My opinion:

I think most knee problems develop from poor or unbalanced muscle growth. Some of it is from the strain on the knee, but a finely tuned surrounding muscle group should inhibit knee problems. My knee problems on regular geared bikes have come from too much growth in my quads, and an underdeveloped iliotibial band. The IT band is not really worked in cycling, but nonetheless vital. It was shorter than the rest so it pulled my kneecap slightly out of wack. Just one example of a knee problem - my point being that it isn't just the stress that makes your knee hurt, it's stress/growth/lack of growth of the surrounding muscle groups that then leads to knee pain. In your case I'm sure your full-leg is extremely well tuned, probably twice as good as both of my legs put together - so I doubt you'd have knee problems. Do it. ~ David