I just thought I'd put my opinion in here as I got a mention.
I have a full artificial leg, therefore an artificial knee. There needs to be a distinction between those who have a knee and those who haven't. I rest my leg on the pedal and work it from the hip, my 'good' leg does most of the work, hence the joy of having electrical assistance to move. I use the throttle to get going and then let the PAS pick up as I rotate the pedals. This maintains movement and balance. If someone has a knee it has to be able to be bent sufficiently to go through the rotation of the pedals. Lower limb amputees have no problem with this, or balance for the most part. There is likely to be less power through that leg but it's not impossible and much better for mounting, dismounting and stopping. Although Oldman is not there yet, this is where he is hoping to get to with his new disability, if I've read it right.
Regarding what it all looks like is a separate individual psychological issue. I've had forty years to get used to my disability and have got to the point where I don't really care what others think when I ride a step thru. Comments are made by those who think a man (read 'real man') shouldn't ride a bike like that, but if I'm having fun and it's doing what I want it to, then why should it bother me? I have drawn the line at pink or purple colours (!) And, although I've considered a trike I'm not there yet. I guess I'll get one of those when I get the cardigan with the big leather buttons! (As a side issue, I think manufacturers and retailers are missing a trick here. The baby boomer generation are here with reduced mobility but still active. And they have a few bob to spend!)
There are many videos on Youtube with one legged cyclists and for the most part they don't wear their prosthetic limbs. They probably are on fixies of one sort or another. It would help when stopping to lean against something to get off and on so that's not particularly practical on a day to day basis. To get me back into cycling I bought a cheap Cyclamatic and then realised what I wanted/needed from a bike having regained my balance. My reading of this thread suggests that the prerequisites are, a throttle to get going, a reasonable amount of assistance and an ability to be able to vary the power delivery. Depending on the physio there may be a need to reduce the rotation and there have been some good suggestions regarding that, but it's the degree of knee flex that will determine whether it's possible, and that's individual. I've not seen it anywhere, but having a fixed pedal that doesn't rotate and can be used to rest the leg on would be another thought if rotation is an issue.