Hey Bob,
This might not be the best/right way, but I just run the spindle in reverse and put the tool on the back side and use the threading wizard.
The spindle index pulse is just for the synchronization, Mach doesn't know what direction it is turning.
I also tried starting in a relief groove on the front side and setting Z start a negative, and Z end a positive so it threads toward the tailstock with the spindle running forward.
Good luck, Standby at EStop !
RC