Hey Tweakie,
I just tried the trial version of sheetcam, it has a postprocessor called mach3 tangential that posts A axis movements.
Im attaching a test file I made, it seems to do what we are looking for it to do, the only thing is that Mach3 seems to have problems running the gcode at normal feed speeds and it moves really slow, at least with no machine attached to the pc (i dont have it ready yet).
You think you could give it a try to see if its just me or a mach3 problem in general? It has a 1" depth of cut so that I could see if the A axis was turning before clearing the material.