Update, I tried these mods to the circuit, it worked on a saturated but I need to get the resistor values dialed in better. The idea is to make the injector firing detection relative to the supply power.
Tapping into the other side of the diode provides a .7 volt gap from system voltage, so there is some built in-tolerance for the hold driver voltage.
Still sorting it out though, not a final recommendation, need to figure out RX and R2, 2n3906 assumed. Changes in blue.
Here is a retrofit of the circuit using ~50k resistors for rx and r2 (again not final), resistor to zener trace has been cut on the bottom side, blob of hotglue would be a nice touch:
Adds one transistor and a resistor to the design if it works.
Weird thing is with R2 and RX at 50kohms, the CPU pin voltage was limited at about 2.5 volts. Is it possible the current is so low that the high impedance cpu pins are doing the clamping?