I have never looked at the plasma cutting screen, but it would seem to me that the "torch off" command could be altered. I don't know how you arrive at M5 for this - is it a standard instruction (it is used for spindle off on a lathe/mill).
If it is standard, then open the M5 macro, and add the movement to beginning of the macro -
eg code "G00 Z50".
This will mean that everytime the M5 macro is called, the script will first move the Z to 50 (or anywhere else you want it to go), then apply the "torch off" OEM
The fact that sheet cam puts in the G00 Z50 again won't matter, because the torch will already be there.