Non-Self vs. depersonalization disorder

Very interesting question! From the link you provide:

• Presence of persistent/recurrent episodes of depersonalization/derealization

From reading the definitions of depersonalization and derealization it does sound a lot like what you might expect from an experience of not self.

• Ability to distinguish between reality and dissociation during an episode (i.e. patient is aware of a perceptual disturbance)

Hmm, this sounds suspect. It isn't at all clear what this should refer to? I suppose it means that someone can remember back to the past and appreciate the difference between that and this new experience of not self?

• Symptoms are severe enough to interfere with social, occupational, or other areas of functioning

The crux of it. Other than seeking solitude for meditation and/or retreat have you experienced any other potential "impairments?" You mention that it seems hard to relate this experience with others. Do you feel a great need to do so? Is there some craving to share the experience with others? If so, what is the motivation for such a craving?

OP: How would you distinguish between the (partial) feeling of non-self as a step towards enlightenment and a depersonalization disorder (or a similar mental disorder)?

Personally, I would compare my own experiences and their effect on my life and attitude with the joyous behavior exhibited by my spiritual heroes. Have a look at this video clip and see the smile on Rinpoche's face. If you watch the whole movie (which I highly recommend) I'll bet you'll get an amazing sense of warmth and carefree childlike playfullness emanating from him. The simple prescription to figure out whether what you are experiencing is authentic is to compare with such people. HHDL is another great example of someone you could look at and compare with. Rinpoche and HHDL are two of my spiritual heroes. Of course, you should decide for yourself who your own spiritual heroes are and compare with them!

If you find that your experience is not conducive to generating in you the same traits you see in your own personal spiritual heroes, the best thing to do is to conclude that you've misunderstood or are having an artificial simulacrum of a true authentic experience. Laugh about it and move on and keep trying to make it authentic. You'll know you're making progress when you compare with your spiritual heroes and see some tiny inkling of a smidgeon of a resemblance :)

OP: (How, in which cases) would you speak openly about non-self-experiences with others?

If you are having the desire or craving to relate the experience, then I'd examine the motivation for wanting to do so. Where is it coming from? What would be the purpose of relating it? What good will come of it? Is it simply a desire to know whether you are on the right track? Is it the craving for praise or a pat on the back? The motivation matters of course and once you are clear on the motivation it might be the case that the answer comes to you :)

Who are your spiritual heroes? Can you identify a mind that you are trying to emulate? Can you see the behavior they exhibit and the traits they seem to possess? Finding someone who you really believe exhibits the traits you wish for is a good step towards answering some of these questions because then you can look and see if you're making progress by comparing. Are your experiences and understanding gained through Dharma practice leading you further towards the traits you are looking to possess or farther away?

In depersonalization and derealization disorders one recognizes a self that one is not, and that one believes one should be. This creates stress and anxiety, along with distortions of one's perception of reality.

With anatta (no-self) one realizes that the egoic self is superficial, impermanent, and illusory, and loses one's attachment to it. This creates peace and openness, and a clear view of reality.

I imagine that as people struggle with the idea of anatta during their practice they might have moments of (non-clinical) depersonalization and derealization — moments where they reject the world or reject the self without releasing their attachments to these things — but anatta is ultimately the resolution of this problem.

We should be wary of what Kew Wilber called the Pre/Trans fallacy. People who are still in a 'conventional' mindset (the ego-identification mindset) often confuse pre-conventional and trans-conventional states: enlightenment can appear the same as infantilization; the experience of samadhi can be conflated with a drug-induced stupor; higher understanding cannot easily be distinguished from mental confusion... It's easy to get our heads turned around.