Actually you could. It's a simple matter of low-pass filtering, or in other words extracting the DC component of the Fourier series, of the injector voltage pattern. And then just data log (record) the voltage. A simple scaling converts it to percent.

You could do it with a simple analog circuit. Take one injector signal, translate the pulse using a convenient (5 or 10 V for example) zener voltage and a resistor, extract the dc component using a simple RC low-pass fliter (1 -2 Hz), and scale that using 100% = Vz (5 or 10V picked earlier). You could use a hand-held DMM reading 5 or 10 volts or one of those low-cost LCD panel volt-meters that reads 5, 10, or 20 volts (100% corresponds to whichever zener voltage). That will give you a real-time readout of dc %.

I'll try to post a schematic in the coming days.