I have noticed that the x7alt is not observed by BIAB very well, particularly with RealTracks.

I think part of the problem could be the confusion with, and differences of understanding of, the alt form. Hence, it is better to write out the chord exactly the way you want it, with all the tensions and altered tensions you desire: G7#5b9, for example.

(My personal understanding of the alt chord is this: Make sure you DON'T play an unaltered 5th or 9th. Raise 'em, lower 'em, or do both, but don't play natural five or nine.)