I have been following this with interest.
Tempo changes are both possible and quite easy when you figure it out, any tempo change is indicated in the piano roll header strip, amongst the bar count indications, maybe a bit subtle but it’s there and can be changed mid-song.
Accel and rit are possible, I have done it.
issue-regarding-tempo-editing
I hope this script can play nicely with tempo changes when finished, even if it isn’t commonly used.