Pulling the ABS fuse created some error codes :)
I looked at the wiring diagrams and my above assumptions are wrong. The OBD port does connect straight to the Ford ECU. I also tried a different scan tool in Ford mode and I was able to get it to connect; I guess that the other one wasn't good enough.
Therefore I should be able to implement a mode that goes straight into the Ford V8 ECU without going via the Gateway ECU as the T4.
Anyway, I have already gotten the Gateway ECU protocol working. These are the screenshots:-
V8 Gateway ECU
http://pscan.uk/features/screenshots/v8gwinfo.png
http://pscan.uk/features/screenshots/v8gwcodes.png
http://pscan.uk/features/screenshots/v8gwlivedata.png
I think that the numbers for Revs and Coolant temp are a bit off, I need to re-check them.
X12 V8 Engine ECU (in T4 mode - via the Gateway ECU)
http://pscan.uk/features/screenshots/x12info.png
http://pscan.uk/features/screenshots/x12codes.png
I haven't finished reverse engineering all the live data parameters (there are lots) yet. I will update when that's done.