I just scanned your new code & it looks pretty generic now...great!
The only caveat, is that when an interrupt happens during a Mark, you may(will) get a corrupted modulation pattern, which some receivers reject (=insert a short space in the middle of a mark). But probalby worth it if users need access to all the timers for other purposes. As a minimum there will be a Timer0 interrupt every ~1ms. When I get back to base next week I will check it out on the oscilloscope, along with the timings and let you know.
Well done!