Good idea to start a thread for this.
The problem is that (for our use) torque levels at the crank are around 3 times greater than at the rear hub - so you need a good quality freewheel.
The best ones seem to be the ENO (White Industries) and the Tensile (Onza).
The ENO 22t is a particularly convenient one to adapt because it has 5 circumferentially milled slots which can be used to bolt to.
So, you screw your freewheel onto the trials cranks and then you need an adapter to mate up with one of the three common chainring BCD standards. 104mm 110mm and 130mm. For my bike, I designed a chainring to fit directly to the ENO 22t freewheel (see photo).