I think it will be extremely complicated to make the WebIF multi-user, the best you could probably hope for is to have it one user at a time. The most practical possibility I can think of is to depopulate the applets available from the front page for specific classes of user, and have multiple username/password combinations that enable user classes (with an additional disable on the reboot).
There are probably many gotchas that will surface. What, for example, if users try to schedule programmes which will conflict when they get added to the schedule at next boot?
Almost anything is doable with enough effort, but the biggest obstacle at the moment is there is only one user that wants this feature. Anybody else?