Aha:
https://smartech.gatech.edu/bitstrea...00908_mast.pdf
Page 15 of above for a diagram of the mechanism... and page 24 onwards for detailed analysis of transition sequence.
The correct terminology for the concept shown at the beginning of the thread appears to be a stop-fold tiltrotor.
Apparently Bell did indeed test a full 25 foot nacelle folding rotor in the 40x80 foot Ames wind tunnel in Feb 1972.
The photos I can see look remarkably like the drawings of the power unit above.
Boeing, Bell and ONERA in France were involved.
They tested stop-fold proprotors up 0.85 Mach.
Wow. There's nothing new under the sun...
and more here:
https://hushkit.net/2021/08/03/bell-...-dr-ron-smith/
A report post the Bell 1972 tests is linked to in the above article - fascinating reading.
https://ntrs.nasa.gov/api/citations/...9720022363.pdf
So, thinking this through.
For transition from aeroplane prop driven mode to jet driven mode, presumably you reach a prop driven speed where wing borne flight driven by turbofan thrust can assume vehicle load - and that still won't exceed airloads on the prop.
Optimal transition speed is between 150 and 175 knots.
At that point - you progressively reduce torque, increase turbofan thrust, freewheel the prop, feather it - it stops in the airflow, is locked and retracted flush with the nacelle.
You are then free to accelerate further - limited only by jet thrust and fixed wing airframe considerations - up to 400 knots apparently.
Transition back is the reverse, feathered prop deployed into airstream (actuators need to be powerful, but still doable) decelerate while still driven by fan thrust, progressively transfer power to engage prop, increase pitch, and you're back to turbofan driven prop mode.
Controls very similar to the Osprey - cyclic and collective that progressively wash out or back in according to mode - and beep-able nacelles.
Corrections or more sophisticated explanations welcomed.