M function during move

Hm, is this example taken from a document? If so can you provide a link to it?

I see. I don't think you will get this done with standard gcode.
Maybe with a python remap?

linuxcnc.org/docs/html/remap/remap.html#...ming-embedded-python

[edit]

There is a hal-pin that might be useful:

motion.distance−to−go OUT FLOAT

Distance remaining in the current move

sure, can't see why not. Do you need this for all moves or just for some?

What about the distance to the end of the path, is that a constant value or does that change?

[edit]
I mean the "t" or "S" value in your picture.

So, if you write a hal component that constantly switches a pin at the desired distance before reaching the end point you could use M64 P0 with an and2 block to "enable that signal to go out and then M65 P0 to disable the output again. Maybe you could even find a way to automatically disable the output after motion.distance-to-go as reached 0.
Your S value could be defined in the INI or in the GUI.

Just an idea.