Since I have to figure out this stuff for myself, I'll try and help the next poor soul that comes around looking for answers.
I finally got Mach to recognize the inputs.
The port number needs to be port #1 (for the inputs at least). That works for Mach input pins 9-16 (GRex inputs 1-8).
Ill wire up the other GRex inputs (9-16) and see where they match up.
No joy on the GRex limits. I'll have to test and see if they are hardware limits handled by the GRex itself.
Mach doesn't seem to recognize them, at least not yet.