I have the relays working for the spindle yes, under the config. I have a signal coming back to mach 3 via Hillbilly BOB, If I go into the diagnostics screen when spindle is running I get a indicator in mach 3 both on input and output.
I'm not quite sure I follow you on the M100, or where to put it, or even what the code would look like.
This is second Hillbilly BOB conversion I've done for Boss 5 machine, but I'm not sure if code has changed or what. More than likely i've overlooked some simple setting but, not sure.