Hi ZeroDrop,
Actually, here's a solution for Oneshot125 on the Mega.
After giving it some thought, I realised I might be able to get it going by using the timers' interrupt service routines (ISRs). I wouldn't say it's a natural fit with the Mega's timers, but it works, producing a characteristic Oneshot125 pulsewidth from 125us to 250us.
The sketch sets up 12 timer channels on the 16-bit timers 1, 3, 4 and 5. It uses the timers' overflow (OVF) ISRs to load the OCRxx registers with the pulsewidth values on the first pass, and reset the PWM output back to 0V on the second.
There are 12 counter compare "cc" variables one for each timer channel. To generate a Oneshot125 pulse on a timer channel, just load the corresponding cc[] array with 1000 for minimum throttle, 1500 for mid throttle and 2000 for maximum, then enable the timer's overflow interrupt. Once the pulse has been sent the ISR disables itself.
The sketch outputs a Oneshot125 pulse set to trigger every second on all 12 channels. The pulse widths cycle from 125us, 187us, 250us then back to 125us and so on...
Please find the attached sketch as a zip file...