Graviman,
I agree with your comments to Shawn, I would guess in the current multi-engine turbine fleet most just use RPM. How diifficult would it be to do? OK some FADEC controlled engines can be Q or T matched but only because they can. Torquemeters these days are pretty simple as well if you wanted to go that way with digital electronics, phonic pickup measuring phase shift
I'm currently based on a ship that has 8 engines the same, diesel electric, doesn't seem to be an issue.
As for FADEC, when I did the PW 206/207 course the instructor mentioned how the PT6 FADEC in the S-76B became rapidly obsolete because it was basically over engineered and in a short period of time you could not purchase the components in the FADEC any more! His comment was that the FADEC in the 206/207 engine was probably very similar to what was in a Fuel Injected Korean car. The box it was in and the screws were made in the US by Hamilton Standard and that was about all.
There was a similar issue with Concorde near the end of it's life. They were having difficulty supporting the analogue and steam powered electronic systems in it.
On another issue with diesels that are out there in FW the penalties aren't as great as you would first assume. The lower SFC trades into not having to carry so much fuel. With the current cab 1 hours fuel = 2.5 average sized people!
Also I think you would need to address the torsional issues well on a diesel.