Open-monitoring meditation versus doing nothing

Short: since good householder wouldn't not do anything (which would be already ending of Kamma, Nibbana) it's most required to monitor all the way, especially in regard of skillful and unskillful deeds. But good to start right by the practice of Sila and Dana first, to get the raw stuff (deeds by word, body) known and abound. The rest comes by it's "own" (given causes).

• there is a form of meditation called 'open-monitoring', which involves paying non-judgmental awareness to everything that arises in the present moment

I think in Sutta terminology this would be close to concentration development leading to mindfulness & alertness.

"And what is the development of concentration that, when developed & pursued, leads to mindfulness & alertness? There is the case where feelings are known to the monk as they arise, known as they persist, known as they subside. Perceptions are known to him as they arise, known as they persist, known as they subside. Thoughts are known to him as they arise, known as they persist, known as they subside. This is the development of concentration that, when developed & pursued, leads to mindfulness & alertness. https://www.accesstoinsight.org/tipitaka/an/an04/an04.041.than.html

but

• I have heard, and read, that Buddhism typically advises one to have some experience with concentrative meditations before engaging in such open meditations.

Thus have i heard;

"Suppose that there is a foolish, inexperienced, unskillful cook who has presented a king or a king's minister with various kinds of curry: mainly sour, mainly bitter, mainly peppery, mainly sweet, alkaline or non-alkaline, salty or non-salty. He does not take note of[1] his master, thinking, 'Today my master likes this curry, or he reaches out for that curry, or he takes a lot of this curry, or he praises that curry. Today my master likes mainly sour curry... Today my master likes mainly bitter curry... mainly peppery curry... mainly sweet curry... alkaline curry... non-alkaline curry... salty curry... Today my master likes non-salty curry, or he reaches out for non-salty curry, or he takes a lot of non-salty curry, or he praises non-salty curry.' As a result, he is not rewarded with clothing or wages or gifts. Why is that? Because the foolish, inexperienced, unskillful cook does not pick up on the theme of his own master.

"In the same way, there are cases where a foolish, inexperienced, unskillful monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. As he remains thus focused on the body in & of itself, his mind does not become concentrated, his defilements[2] are not abandoned. He does not take note of that fact.[3] He remains focused on feelings in & of themselves... the mind in & of itself... mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world. As he remains thus focused on mental qualities in & of themselves, his mind does not become concentrated, his defilements are not abandoned. He does not take note of that fact. As a result, he is not rewarded with a pleasant abiding here & now, nor with mindfulness & alertness. Why is that? Because the foolish, inexperienced, unskillful monk does not take note of his own mind.[4]

"Now suppose that there is a wise, experienced, skillful cook who has presented a king or a king's minister with various kinds of curry: mainly sour, mainly bitter, mainly peppery, mainly sweet, alkaline or non-alkaline, salty or non-salty. He takes note of his master, thinking, 'Today my master likes this curry, or he reaches out for that curry, or he takes a lot of this curry or he praises that curry. Today my master likes mainly sour curry... Today my master likes mainly bitter curry... mainly peppery curry... mainly sweet curry... alkaline curry... non-alkaline curry... salty curry... Today my master likes non-salty curry, or he reaches out for non-salty curry, or he takes a lot of non-salty curry, or he praises non-salty curry.' As a result, he is rewarded with clothing, wages, & gifts. Why is that? Because the wise, experienced, skillful cook picks up on the theme of his own master.

"In the same way, there are cases where a wise, experienced, skillful monk remains focused on the body in & of itself... feelings in & of themselves... the mind in & of itself... mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world. As he remains thus focused on mental qualities in & of themselves, his mind becomes concentrated, his defilements are abandoned. He takes note of that fact. As a result, he is rewarded with a pleasant abiding here & now, together with mindfulness & alertness. Why is that? Because the wise, experienced, skillful monk picks up on the theme of his own mind." https://www.accesstoinsight.org/tipitaka/sn/sn47/sn47.008.than.html

open monitoring would be closer to meditation of the unskillful monk

I think you're misunderstanding the word "concentration" as it applies to meditation. This is very, very common and I can imagine that it's wrecked the practice of thousands of people. I want you to think about holding a piece a paper up against the wall with your finger. How much pressure does it take to hold it there? If you applied the energy most meditators try to foist on their meditative objects, all the blood would drain out of your finger due to the pressure being exerted. You'll also quickly grow tired and maybe end up hurting yourself. In actuality, of course, it takes almost no effort to hold that piece of paper in place. All you have to do is keep your finger on a single spot, gently apply it, and keep a very minimal amount of sustained pressure.

Single pointedness, applied, sustained. Sound familiar?

So what am I bringing this up? These supposed "open" meditations that you are talking about follow the same formula. Even "doing nothing" on the cushion works the same way. I'm sure you've read about shikantaza meditation or the art of "just sitting". Let me be the first to tell you that "just sitting" is anything but. Rather than get into the intricacies of the practice, however, let's just say for now that act of not moving satisfies the criteria of single pointedness, applied thought, and sustained thought (and if you don't believe me, sit in one place, utterly still, for 30 minutes). As we pin down our bodies according to those restrictions, only then can we really "notice" the distractions and other phenomena that arise. We have to establish a stable seat in order to appreciate those things that would try to move us.

So those open meditations? They aren't really as open-ended as you might think. They only really work provided that we've first "set up our seat" so to speak and have achieved stability of mind (not, as most people think, a laser-like, "concentrated" mind). If you just plop down on the cushion and start noting what comes up, you'll quickly start following your mind down all sorts of rabbit holes. Worse still, you'll have no place to return to for when you do wander. What comes up in meditation is actually pretty meaningless. The coming back to rest on that single place is what really matters.