Slightly better work around:
mc.mcCntlGcodeExecuteWait(inst, "G1 X10 Y10 F450")
mc.mcCntlGcodeExecute(inst, "G1 X-10 Y-10 F450")
This will perform the rapids as expected.
Caveat: Running above code from the LUA editor, if the motion is stopped with the on-screen stop button, no amount of coaxing would make it run again short of closing and reopening MACH4. Might work better saved as a proper macro. I stopped messing with it at the above work around.