Phil,
Don't know what to tell you as i have never fooled with doing or modifying any post processor. I would think that
OneCNC should provide you one which is done for Mach Mill and Lathe.
Just one of those things you learn from experience, in that, the CAM program may not be worth much if you can't process the information properly for the recieving program. Some CAM programs charge you for one.
Try a post processor for a Fanuc lathe as that may do the trick.
I don't use OneCNC and haven't even looked at their info on the program, but, CAM programs sites usualy tell the PP's they have or give you specifics on the programing for changes. If they advertised that it posts to Mach then you have ever right to expect something from them IMHO.
Sorry can't be of more help.
RICH