I've been asked about controlling the spindle on/off through mach 3 before and
I found this board that looks like it will do the job pretty easily.
CNC PLASMA ROUTER RELAY CONTROL4 STEPPER MOTOR SYSTEMS
at
http://stores.ebay.com/HUBBARD-CNC-INC
It still won't solve the problem of having to reset the speed control back to zero before it comes back on again but it will certainly turn it on then off after a first run. Shouldn't be much of a hassle anyway unless you have a tool changer, ha.
later Hoss