I went and made a little transistor switch to indicate when the injector is grounded.
This is the schematic:
When I started the engine, I looked at the oscilloscope and saw this:
So it works! The dots you see correspond to the "B" part of the current graph:
I turned on some loads for the engine, and, not suprisingly, the dots extended to about double their length. So the dots are an indicator of the pulse width of the injector.
I need to see now if the guino can register that as the injector length, or if it will only look at the first part, and ignore the dots. I think some tweaking of the "injector delay uS" is in order.