I was actually under impression that Adobe will be committing code to the compiler and / or framework developed here - that was at least my understanding of the last whitepaper posted by Adobe, or am I misunderstanding it? Maybe the word "support" isn't the best choice though.
In my experience, the mx_internal namespace was used often for the purposes such as to cover the weak points in design. For instance, have you tried to implement some sort of collection-view interface and extended Proxy, you would've known that the framework code insists on the objects inside the collection have mx_internal:uid or even mx_internal_uid property :) I think that the problem of not documenting things could've been solved by @private ASDoc meta (and it was indeed used all over the place) and it won't be particularly difficult to have that sort of comment / annotation in HaXe source code to show that the API is experimental. So, I think that this is a non-issue.
