[Granitehill]

Thanks for the simulation and test data - this will be very useful.
I'm probably going to use 2N4416A FETs - I've got a bagful, so I can select a couple for low Idss (5-15mA spec range). The value of the drain resistors is due a bit of experimentation as well.

The unloaded Q of the resonator I expect to be in the 300-400 range. I've got some excellent (large) silver-wound ceramic coils harvested from old Clansman radios. The variable C will be the capacitor and gearbox from an old BC221 - which I guess is still one of the best precision tuning mechanisms ever. Since you mentioned the use of a T50-6 inductor, I want to try that as well just to get an idea of the comparative temperature stability. It'll be worse, obviously, but just how much will be most interesting.

I think output coupling will be straightforward - I'm not constrained by low component count. Probably a very high impedance source follower with a few pF coupling to one of the drains. A low-pass filter will be on the output, so harmonic output from the oscillator isn't too much of a concern.

Thanks again for all the pointers - I'm now a lot more comfortable that the Franklin is a viable VFO, given some care and component selection. I can see why it's probably not an optimum choice for a production design, though!