I have been working on this quite a lot recently. Time for an update I think, and also another plea for help. Working with Greeners was great but I can't get hold of him anymore.
1. I now have a real T4 which is hugely helpful as a development tool, and I have had a chance to try it on a 260.
I found that it would go into the engine ECU, but attempting to read error codes causes it to go into a loop that you can't get out without shutting down the T4.
Reading live data works.
2. I have also found that I can get straight into the engine ECU using Ford Mustang commands. The emissions based data is already done (as in the same as an OBD2 tool). This needed a bit of tweaking compared with OBD2 but I think it works.
3. The amount of data that can be extracted in Ford mode is insane. There appear to be far more parameters in there than I have ever seen on any other ECU, with sensors for things like fuel tank pressure as well as level. You can check for errors from the fuel filler cap sensor. I don't know how many of these will actually work on a V8 R75/ZT. It might be that the sensors are not there, or the ECU might have different firmware to a Ford Mustang. This is certainly data that a T4 can't read.
I can also read error codes reliably :-)
At the moment I am wading through all of these parameters and converting them all, and I don't even know if they will work. I have to travel a bit to meet up with an owner and I don't want to keep hassling him so I am currently doing the whole lot and hoping that I'm not wasting my time.
If anyone else is closer and can let me have a play with my results so far it would be really useful.