It will probably work as is. You are looking at 0.7 milliamps (tiny) with the standard 100k resistor on the mpguino schematic. The processors are pretty cheap ($4.00) so don't over engineer it too much, except to make the processor replaceable
re: fuel pressure, do you know what causes it to change? The other variable pressure schemes I've seen were to keep the pressure drop across the injector consistent and thus did not need accounting for.