I'm genuinely interested in hearing more about the non-starter. I've stripped hundreds of starters over the years and every time I thought, there must be an easier way of doing this!

I think you would seriously struggle with an open ECU. There are multiple reasons for this but the two main ones would be:

1) Manufacturers charge a ton for mechanics to use their software to acces their ECU and there is a recurring subscription charge for said software so you would be competing against a very monopolistic market.

2) Every car is drasticly different. A universal ECU would need to be a remarkable feat of engineering for it to be able to handle a honda civic as efficiently as it handles a Ford F150. Considering the amount of responsibility ECU's now hold in a vehicle, creating one which can handle parameters for all cars would be almost impossible and probably ridiculously expensive.