It can be done… I did t in the park on crutches though I wasn't camping, absolutely killed me though and a lot of it wasn't much fun. Can't speak for Glastonbury, we camped beside a guy at Reading one year who was on crutches and he was having a terrible time of it with the softer ground.
Depends on the injury really, there's a really good chance of being accidentally knocked or tripped, I went over twice at t in the park just because of people not being aware of me, so if the knee's still delicate it might not be a good idea.
Sorry to be a bit negative but I don't think I'd do it, not if there was a likelihood of causing further injury. I think I'd end up playing it so safe I might as well not be there.