dhenry:
How about the idea with fixed 32 bit buffers and shift them out at a fixed frequency?
How does that solve your problem?
I meant shift the 32 bits stream to a RC filter. I didn't investigate it, but I think the Atmega has a hard job with 12 outputs at PWM with a reasonable frequency. Therefore I was thinking to preprogram the levels (to decrease the processing load) and decrease the counts to 32 to be able to raise the frequency on 12 outputs to > 500 Hz (preferable in the kHz range).