Oh ok, So I wasnt off base then, whew, thought I was seriously confused there for a moment! 
I think your idea should work even with the signal generator’s existing output abilities,
I just looked at mine and both the frequency and the duty cycle are fully adjustable.
I am wondering if you realize you can adjust it down to 10Hz increments?
On my version Nano you can anyways, when you have it on the FR menu the right or
left keys adjust the frequency, If you want to adjust frequency more precisely, Holding the menu key
will move the cursor from whole KHz resolution to .1KHz and .01KHz resolutions.
It wont give you pulses of frequency, but the steady signal to the injector should give your pressure
gauge more time to settle on a more accurate reading anyways, no? ( though I guess you could flood
your cylinder with fuel pretty rapidly doing this) 
Happy testing! Have a good one, Troy