>>how about some clever (meaning cheap/simple) ideas for PIC
>>sense zero crossing from the transformer secondary.

AN521, "Interfacing to AC PowerLines"
works like a charm.  I have 2 projects where I did just that and they
have been working fine for over a year.

The only real reason to sync to zero cross is if you are using SCR or
Triac as the current switch.
Then you let the zero cross turn off the device.
If you are using MOSFETs, then you can turn them off anytime.
So, set the PWM frequency much higher than 60 Hz and don't worry about
sync.
This does not take into account switching transient noise if that is a
concern.

Hope this helps,  Gary Skinner,  ESI