Present OBD design has three pins left after CAN, K line, ethanol content, ethernet, tachometer/crank trigger. Two are outputs, one from 3.3V DAC, other through 9A low side drive. In the present form factor, there is not the space or thermal capacity for further high current outputs even if an extra connector were added.
External ignition/injector amplifier boxes are available reasonably or one can be built. 6 outputs in a different form factor.
You could output fuel mass or duration, DI/PI fraction on CAN and get three or six cylinders into one CAN packet.
Thought needed for when you would output new data and whether that one cycle delay between PI and DI is acceptable, because the PI fuel for a combustion event is mostly delivered before the DI waveform which is susceptible to change. Also multiple injections in DI complicate it.
It needs more thought. I certainly like many elements of it.