I could see how this could be useful. But the way grbl works it could be a nightmare for a software developer. It would next to impossible to ‘debug’ something a user put in the stream in no telling how many places. Just supporting the original code would be problematic.
Gcode send a code for on and another for off for every ‘dot’, generates megabytes of data. You want to insert code in between these.
Sounds like a major bottleneck, but @LightBurn will have to comment.
Good luck 
