People do curls at the squat rack all the time. It's easy to rest the bar without having to drop it, and the really serious weightlifters at this gym in Gainesville I used to go to would exclusively use the 45 lb bars for curls, using at least a 45 lb plate on each side, and finishing the curl far above their heads. That's probably not the best form, but is that really the point?
Serious lifters don't curl in the squat rack- they fucking kill curl-jockey bitches and eat them for clogging up the shit that they need to use.
Bitch.