Yes, thanks for pointing that out. I later found that in the Marlin Laser Configuration Guide (2.0.9) after I read your post.
What was confusing to me is how you would implement this in LB. It occurred to me that you might have to manually edit the Gcode and replace M03 with M04? Which led me to …
So it turns out that LB Inline mode doesn’t use any M03 I (or M04 I) commands in its gcode output. It puts an Xnnn Ynnn I Snnn string as part of any G0/G1/G2/G3 commands, but it never actually turns the laser on or sets it to Inline Mode.
I added an M03 I start code (and an M05 I end code), and now the laser turns on and off when it is supposed to (more or less - there’s still something odd going on where it seems to turn off briefly mid-pass).
Poorly written software for something they charge for (IMO).
I’ll play around a bit more, and substitute M04 I instead of M03 I in the start code, and see what happens.