I recently implemented a machine Pause (grbl M0) for the two fold purpose of:
- pausing after a calculated continuous runtime of cutting to allow the air supply pressure to ‘catch up’ when the system is undersized for SCFM use. and
- pausing to allow a shape cut to be removed from the table.
I liked the ideas discussed in this thread and the cross linked thread, thanks all for contributing.
The grbl M0 pause requires operator invention to resume, either click the software program Cycle Start button or a physical Resume motion pushbutton on the CNC controller panel. The .scpost code is implemented in OnPenDown() so the pause takes place at the pierce point of the next cut, and includes a test for having just finished cutting a part; the next pierce position is nice because its over the sheetmetal in theory, as torch cooling air continues to flow 
The torch is also retracted to safeZ during the pause.
See the scpost here: https://sheetcam.discoursehosting.net/t/grbl-plasma-w-thc-torch-touchoff-and-rotary/7342/15