[projectRDM]

Excellent idea, but as you already mention there are questionable issues. I'm not certain of the coilpack signal's output, but I'd assume you'd want to use a 6A diode to isolate each signal to protect everything. Diodes that large get very hot, and would need to be mounted somewhere in the open since they'd be working 100% of the time. That alone would suggest you'd want to build a subharness with the diodes prewired in and a breathable casing or heatsink around them, pushing up cost, if you plan to offer it to the public. I only say that because 95% of the potential buyers will NOT follow the proper instructions and wrap them up with tape or lay them across a water hose somewhere. That's a given, and then you're on the hot seat having to replace parts and kiss ass. I learned a few years ago not to offer anything unless A) you can back it up 110%, or B) you offer as is without any sort of support whatsoever and they know it.
Secondly, the longevity of a diode in that application is hard to determine. The OE harness uses diodes in a few places (the headlight circuit for one) but it's a high current sealed unit that's inside the car. Outside under a hood with water intrusion, NVH, and the added heat would play a heavy part in keeping it alive.
As far as the actual testing, you're chartering new territory. I personally have only touched SOHC engines and parts to remove them and core the parts out for scrap. I don't think I've even opened a SOHC FSM before.

But it's a great idea nonetheless and would make the above average installer eager to skip that one extra step of tracking down a DOHC cluster or tach, as they're becoming more scarce every day. To the rest of them you'd just plan to expect some hand holding to walk them through it.