Not currently I think. But doable. Interesting idea. Fancy creating a feature request issue here:
As you say, part of the problem is the grouping of Logoff with Reboot and Shutdown. We could have another independent issue to provide some separation here which would be a far easier / smaller issue/feature as it would only concern the Web-UI layout rather than extending our user model within the database to accommodate shutdown/reboot privileges and dealing with who can create what user etc.