Hi Dion,
Based on what I do on a Fanuc 18i control at work this is my experience,
the biggest problem you may have is that the code must be written in the correct format for the rotation to work, e.g. every line must have an X and Y position even if the position of one axis is the same.
Some controls also require you to cancel the rotation before each tool change, also certain functions can not be used e.g.
Changing from G54 to G55 etc. without first cancelling rotation.
I can not tell you how Mach uses this function as I don't yet have a mill, just a Lathe (rotation is not very useful on a Lathe)
You will have to ask Art how Mach3 implements this, if at all.
Regards
Graham.