My current avatar shows my Hase Lepus recumbent trike that I've had for about 5 years and I'm just getting round to trying out some assist on it.
I've bought a Q100 328rpm 20" front wheel from BMS, as that would seem to be the simplest and lightest way to achieve moderate assist, up to about 18mph at 36v.
If I ever want more oomph, it looks as though the BB02 would fit nicely on the front boom though, and I would still get all 27 gears of the "mountain drive" because they're achieved via an intermediate shaft under the seat, with the 9 speed cassette, and a 3 speed cluster on the rear half shaft. I'm just a bit concerned at the strain on the 7ft chain with the present BB02 PAS assist lag when changing gear.
I'm thinking that the front Q100 250w motor will do quite nicely up to 18mph though, as the Hase is so easy to pedal at 15mph with the low wind resistance.
I'm surprised that you don't hear about more recumbents (bikes and trikes) being assisted, the meagre UK "legal" power ratings would go that much faster with them.
I've answered my own question there though. I suppose. You'd have a job keeping them down to 15mph if pseudo legality was the goal !