It may sound weird, but is it possible to map a (separate) key to accelerating at a percentage instead of full-throttle?

I don't have the money for a decent wheel/pedal set for the time so this would help even with Formula BMW, never mind a beast like FZ50 or F1.

Try "button control rate" in the options menu. It's not exactly what you're looking for, but tweaking that value combined with careful button press modulation can result in the desired effect.