Maybe this is just crazy talk, but couldn't you try and generate your sync signal with a bi-stable oscillator, you'd want to watch that with the MCU so that you don't keep sending data as your sync goes off, if not using the oscillator directly, you could use it to keep track of were you are in your output (send the sync with the MCU, but use the oscillator to time everything right.
(Please excuse any tyops I've made, it's late, and it's hard for me to keep up with my high standard of typing when I'm tired.)