You might try a spunding valve. Ferment down to some gravity, then attach valve set to desired pressure. If the pressure is too high, it will just vent back to the set point.
There are people who use these for pressure fermenting, but I'm not sure what the upper limit is without stressing it out. I suppose the key would be knowing what gravity, relative to expected FG, would result in the correct volume of CO2. At 60°, you would want to be at 16 psi for 2 volumes for example.
You could use a cut dip tube to avoid the yeast layer, then transfer under pressure to the final keg. I would worry about it plugging with a normal length tube.
I think this would work much better on a conical where you could vent the yeast plug out the bottom, then transfer.
If you used the keg as a secondary, well above FG, then the yeast would be much less if it didn't stall.
I've been thinking about something like this too, but I haven't done enough research on fermenting under pressure yet. The question I'm trying to answer is: what's the relationship between gravity points and volumes of CO2. It should be pretty easy to figure out, I just have not done it yet.