If someone has a cast or assisted walking boot and can't stand for long do they have handy cap lines that are shorter?

Depends on the attraction. In most cases, guests can get a special assistance pass and enter thru the exit for faster boarding.

Some rides I wouldn't recommend getting on in that case, there's not enough room and are awkward to get into even without a cast, but yes usually you can get on though the exit with the assistance pass. For example if you are in a wheelchair you can be helped into the ride.