Is a Theravada Buddhist permitted/able to take a Bodhisattva vow?

The Buddha did NOT take the Bodhisattva vow. His vow was to both reach enlightenment and provide a way for any other being who wants it to also reach it. He did both things. These thing are not the Bodhisattva vow by any means.

The Bodhisattva vow is contrary to it, one takes a vow NOT TO REACH ENLIGHTENMENT until one has helped all other beings to be liberated. This is mere attachment to becoming and to rebirth. First of all, beings are infinite, and vowing to liberate them all is non-sensical. Secondly, as stated by the Buddha, no one can liberate other beings. The Buddha provided the path, but cannot walk it for anyone. Thirdly, the vow would want for you to give others what you yourself do not have, have not realized and experienced. So even if it were possible to liberate others, which it isn't, you would still be unable to do it unless you yourself become enlightened. A Bodhisattva is by definition unenlightened, and so does not stand a chance.

Because of this Bodhisattva doctrine, which is contrary to Dhamma, no one from the Mahayana is ever liberated, and instead clings to becoming and rebirth, out of fear of letting go.

The bodhisattva vow came from the Shakyamuni Buddha, he made the vow to become a Buddha under the Buddha Kassapa. therefore no school of Buddhism should decline a student just because he/she wants to make such vow, the point with Mahayana is that the main goal in that school is to attain Buddhahood, therefore the vow is very important.

For Theravadins the main goal is to become an Arahant, therefore the vow is not crucial. Don't forget that the next Buddha to teach the Dhamma (Maytrea) will only come after 2.500 years once the current Sasana ends, so we are talking about a Paccekabuddha (Private Buddha), a lone Buddha that does not teach the Dhamma.

The term theravāda means "doctrine" (vāda) of the elders (thera). It generally refers to the teachings found in the Pali Canon, commentaries, and sub-commentaries. Anything laid down as doctrine in this large body of literature should be considered as theravada.

Does Theravada have scope for individuals to even make the Bodhisattva choice at all when the time comes?

The Buddha did not, as far as the theravada goes (and as far as I'm aware), teach his followers to become Buddhas, since his focus was on leading beings to freedom from suffering as soon as possible. It is acknowledged as a valid path, however, since that is obviously the only way a Buddha can arise. In many places the Buddha talked about how great was the arising of a Buddha in the world, for the benefit of many.

Even if it does, does it then allow for individuals to make a promise, in advance -- i.e. to vow -- that they will make that choice at that time?

Again, there is no doctrinal teaching on the matter; the closest you'll get is in the Jataka commentary, translated in Buddhism in Translations, which talks about the perfections and characteristics of a Bodhisatta.

Finally, even if Theravada sees both choice and vow as being possible, does it regard people who make that vow as remaining as Theravada practitioners, or would they be seen has having moved into Mahayana?

The theravada doesn't recognize the term "Mahayana"; it recognizes three paths:

1. sammā-sambuddha - one who becomes enlightened by themselves and is able to teach others to become enlightened due to their limitless knowledge.
2. pacceka-buddha - one who becomes enlightened by themselves but is unable to teach others to become enlightened due to their limited knowledge.
3. sāvaka-buddha one who becomes enlightened through the teaching of a sammā-sambuddha.

While thus I lay upon the ground,
Arose within me many thoughts:
"To-day, if such were my desire,
I my corruptions might consume.

"But why thus in an unknown guise
Should I the Doctrine's fruit secure?
Omniscience first will I achieve,
And be a Buddha in the world.

"Or why should I, a valorous man,
The ocean seek to cross alone?
Omniscience first will I achieve,
And men and gods convey across.

"Since now I make this earnest wish,
In presence of this Best of Men,
Omniscience sometime I'll achieve,
And multitudes convey across.

"I'll rebirth's circling stream arrest,
Destroy existence's three modes;
I'll climb the sides of Doctrine's ship,
And men and gods convey across.

-- The Story of Sumedha