the tilt of my upper body doesn't have an effect on my lower body. When I switch from drops to bullhorns, etc, I don't notice any difference w/ my knees. Such forward bars would hurt other body parts more so than it would the knees.
EDIT: NEVERMIND, I just realized you are probably referring to the fact that it would be easy to bang one's knees on the bars in that position. I didn't think of that