Limit the number of Assignable users

Would it be possible to limit the "ASSIGNED" list to a role or to support users that are pre-assigned to a Client? In my situation, I have about twenty people that are the support for the site with over 10K users.

Since not all of the users of the site will be called on to provide support, it would be great to be able be able to shorten that list of users that can be assigned.

If there was an option to select a role (JobTrack Support), or pre-assign support users when you are creating the Client, that would be a big step in answering several issues that are starting to appear as the JobTrack base grows.