So I'm pretty sure it's not just the Fuel Flow and DME, you can't turn the knobs on the ADF either.
Alabeo offered a free plane from their fleet, and I accepted and got the C421, because it had the DME. It kind of works, though I was unable to turn the knobs on the DME either, and it leaves the visual artifact higher up in the radio stack, so meh. I used the C421 ADF as well, and while the knobs won't turn, there are some click points working to turn the knobs.