Most students I've met have fewer problems descending and a lot more problems ascending. What we need are exercises to work the muscle groups responsible for the ascent -- and I don't know any particular specific ones. Any trainers want to chime in?
Another ballet exercise is to hold onto the back of a chair (for safety, not for a bicep workout!) pick up your left foot, and slowly rise to the ball of the right foot and come down. "Eleve" or sometimes people call it releve, if it's preceded by a knee bend. (be sure to switch feet!) Speed is not important. Focusing on the muscles working is. This is a great exercise for your ankles.
As a personal trainer, I thought I'd give my 2 cents on this.
I know you said plies and don't do squats - but what about plie squats? I really like these and you can go as deep as is comfortable for you - but it really forces you to use your inner thigh muscles to 'pull' yourself back up.
I usually use a dumbbell held dangling between my legs or a plate weight.
To be honest though, the same muscles are responsible for the ascent as well as the descent. The same muscles that are firing on the way down are responsible (in some way - whether as stabilizers or synergists) for the return action. Although, if you had to be specific and pin point an EXACT area, I would say I 'feel' it more in my butt on the way up.
I don't think for most people it's a strength issue - I think it's a stability/flexibility issue - or, at least, I think that's MY problem.