I noticed those fixture offsets before, but didn't know the purpose. Now I'm guessing that the appropriate use of fixture offsets would make my Gcode programs less parametric and improve readability?
However it probably won't fix the Mach3 inability to feed hold inside of a subroutine. If I can't feed hold a machining operation, then it must be restarted and the day gets longer. My Gcode parser flattens out both local and file subroutines so the Mach3 feed hold can be used.