Macro Scripting won't get you there either(;-) A script cannot interupt the buffer load where as a variable is open to change right up to the point that the steps are loaded into the buffer. It is a s close as you can get and not be realtime. Sometimes you get very strange reactions when you mix scripting and Gcode together.
Parameterics and #vars are pretty much bullet proof in MACH3.
NOW to help explain what they were chidding you about , here you asked a specific question and I gave you the top answer right off the bat BUT you wanted to change it twice to something else first to brains then to scripting. WE hear this all the time so I just referred you to where you really needed to go/be. BUT you still resist the force and want to do it any other way but the suggested way.
SIGH, (;-)
SO lets readdress your original QUESTION.
NO YOU CANNOT DO THAT in mach3.
Just a thought, (;-) TP