I've had a look at Toyota EFI boxes. If I recall they use a Fujitsu 48pin part that is reputedly a 8051 core. Since the code is in rom, I'm not sure if you can get external access to it. Therein lies the problem. The other problem is getting specs on the micro itself so the code can be made sense of. Personally, I thibk it would be easier to put another EFI box in there. Have you looked at the Megasquirt and its derivatives?

The boxes I've looked at are for the 3SGE and 4AFE engines. They are quite brain dead as far as the latest ECUs are concerned - I'd guess at around 4-8k of code is inside. Even once you've got the code, I doubt these cpus would have the ability to run external code or the pins available to do this. I've had better luck with the Mitsubishi boxes!