Not too many character keys left: B, J, K, N, P, & W.
Nor special characters: <, >, ;, ', \, ?, =, & comma.
Here are some options:
Remove crop: K (kill), Ctrl+X
Rotate crop: \, Ctrl+[, <, >
Toggle constrained crop: =
For me, the Rotate Crop key is going to be the most important one. I don't feel strongly on which key should be used, but I do strongly prefer it to be a one finger keyboard action (e.g. \ is better than Ctrl+[).
Deven