Piggyback is an accurate description.
I'm designing for 8psi, but that's more catering to the limitations of my motor than the capability of the Arduino. The limited number of available output compare interrupts and timers has limited the amount of timing delay to < 15 degrees. I've yet to document these details and don't have the time to explain right now. I will note that increasing possible timing delay would be possible by implementing some form of buffering but my initial attempts at doing so failed. And since at 8psi I don't expect to need anywhere near 15 degrees of delay I stopped working toward it.
Check out my most recent post at the posted link for info on load calculation. In short there is a lot of spare time. 2JZduino is currently also driving an LCD to display engine RPM, MAP, and AFRs.