I'll post more on Tuesday (I'm on holiday at the moment so no internet!) but I'm pretty sure mach3 will do the PWM. As ecat said the period for the ESC is 20ms, so 1/0.02=50hz. This next bit may be wrong, but anyway...running mach3 on 25khz kernel speed with 50hz PWM gives 25000/50=500. So that means you get 500 'steps' between 0% and 100% duty cycle. The ESC is probably 255 steps anyway, and if it's 1023 then it's not really an issue.

You'll probably want to put an optocoupler between the parallel port pin and the ESC just to be safe if the ESC doesn't already have one. Oh also if mach3 is 0 to 10v then just use a voltage divider to reduce that to 0 to 5v. Two 10k resistors should do the trick, or anything really it's not critical.

If that doesn't work then I'd go for the PIC option. I've had a PIC controlling servos before from a potentiometer...it shouldn't be difficult.

Jonathan
PS Have a look on eBay for servo testers. I think you'll find them cheaper.