DanO - after 10 years still nothing........
1/ make sure for replace mode (region punch is selected)
2/ all other recording settings are ignored when replace mode is enabled
3/ in this mode - when you press record a new midi region is created at the first midi note input for however long there is overwriting midi data, if you record longer with no midi input the region extends without midi data in it - that region is then be reduced WILL NOT replace any notes the region overlapped.
This sounds like exactly what you want - take a look at below. What I did was
I have 8 bars of quarter notes.....
I then (with replace mode enabled and region punch selected (very important) - I started recording.
When I get to measure 2, beat 4 - I started to record eighth notes in place of the quarters for 3 beats ( Bar 2 - beat 4 and Bar 3 beats 1 and 2 )
I then let the recording continue without any midi data being recorded for a few beats.
when I pressed stop a new midi region was created with only the midi notes I want to over write the existing ones and the midi region did not extend beyond where the last midi input note was done.
I think this is exactly what you want... please advise.