I was always under the impression that the TT was also build around the A4 chasis. Maybe more similar to the NB then the G/J. Perhaps I'm wrong but I know of many a folk who have taken the 1.8t from a wrecked TT to move into a B3 Passat (syncro - drool).
What does the TT cluster look like? I bet you could use the software from a diesel cluster in the TT cluster. And the EPC light on the gassers is the glow plug light on the diesels... they're connected to the same line and if you drop a gas cluster in a TDI (well at least mine) the EPC light works as the glow plug light.
Perhaps an Audi Allroad would be a better candidate?
Regarding the cluster...I've only seen pictures, but it looks like a standard MFA cluster from a Jetta GLX. Anyone have more info? I'll hit the ETKA tonight and see if I can find the part numbers for both cars. If they match, then I should be able to swap in a TDI cluster.