What's a good policy for hiring substitute librarians and should such a policy include feedback to those substitutes after they are hired?
Our library does not review substitutes as a matter of policy, and in fact we do not review employees who work fewer than 20 hours per week unless they ask for a review. We would definitely be prepared to review a substitute if it was requested - it's not in our policy but I would not have a problem adding it. The difficulty in reviewing subs is that their duties are limited to a small range of tasks, and they are not closely monitored. It would require an adaptation to the evaluation form we use for regular employees.
I will also add that I would be impressed with a substitute who asked for a review. If you are subbing for a library that does not offer it per policy, you should not be afraid to ask. Feedback is an important part of professional growth. I am now a Library Director, and I sent out review forms this month for staff to review me (sent anonymously to the board, who will filter the information to me) - you can't fix what you don't know you are doing wrong, and it is always nice to know what things people think you do well.