The only problem I see is the number of bits of resolution you need. I assume you are doing lights, so the phase-angle vs. brightness is not linear. I also assume you have a zero-crossing detector somewhere. If you can deal with 20 "steps" or so, I would:

Sit in a loop until you get a zero cross
Start a 410uSec interrupt
In the ISR decide which of the 5 channels you want to turn on
After 20 interrupts, go back to the main loop and wait until you get another
zero cross.