Yes.
Make sure the self-reg form has the enabled field statically set to true.
Set the lifetime of the account for 1 hour for example. Make sure RADIUS CoA is set-up so you can disconnect users if this time is reached.
Allow sponsors to change the lifetime of the account on approval.
This should work but may cause a disconnect to the user when the initial lifetime expires. MAC caching should mean a seemless reconnect though.
I'm sure there is a guide on this type of set-up but I cannot find it at the moment. Somebody else may know where it is.