How about a new machine RP1-RP4 on the MD.
An RP machine would work on ONE TRACK like an R and a P work on TWO tracks currently when recording to RAM.
For example, you set track 1 as RP1. You sequence it as you would any other track and whatever is in RAM1 for RP1 will play accordingly. The ONLY difference is that if you have the FUNCTION key down when it is triggered (OR you manually trigger it with FUNCTION down) it goes into RECORD mode (optionally muting the playback of itself as happens now with R and P machines) and records for whatever length you've configured it for. Once recording is complete it trigs itself again as appropriate via sequence or when you hit the key without FUNCTION.
This would save a track every time you use an RP instead of an R and a P - allowing me to have 4 record/playback tracks using only 4 tracks instead of the current 8.
Please!