In the plans to release 9.2, there are good news for Proxy features (such as transparent Proxy SSO), however, you could still add new features for 9.1 ... would be interesting to have profiles for control block extensions ? :-P
Web Security: Exceptions for Extension Filtering by User/Group
Web Security- Exceptions for File Extensions AND Site
For example, I would like to block an extension to a group of users and release a certain type only for a user from the same group.
It would not be necessary to create a new profile of proxy, just add a proxy more granular exception.