The trouble with REX loops is that you really cannot use recycle on all types of loops. It is best for drum loops, arpeggios, or notes played staccato. Anything with transients gets brutally murdered in REX format.
THAT is why nobody has done it. What you need is a transparent time stretch algorithm. I think we're getting closer to that, as it requires a lot of processor power to accomplish. Until then, we do what we can.