Allright, a few tests:
I added the $32=1 line in the VCarve PP:
which generated the expected G-code for a random test cut I created :
But then indeed Carbide Motion rejects the G-code file upon loading, reporting a Syntax error on the $32=1 line.
I then loaded the same G-code file in CNCjs, and it worked correctly (apart from the fact it displayed a weird “error 22: feedrate not set”, but it did not seem to have any impact, the air cut went fine and I was able to verify that $32 was indeed set to 1)
Conclusion: it seems CM does not allow for GRBL $ params to be modified from a G-code file.
I also tested adding the forward slash (which works when using the MDI tab, to tell CM to send the command straight through to GRBL), and the same Syntax Error is reported.
Not sure whether this behavior in CM is on purpose or not, maybe @robgrz can comment.