OK so this is mostly theory so no screenshots but try the following......
Self registration page creates accounts with a logon lifetime of 1 hour.
Amend the Clearpass service so that upon successful logon (after the self registration) the Endpoint entry is updated (in an Enforcement profile) with an attribute to say they have used their quota. (You would need to add this attribute under the Administration -> Dictionaries -> Attributes section for the Endpoint entity).
The Clearpass service which allows the login is amended to check that the attribute doesn't exist before allowing access. If it does exist you know the MAC address has been used previously and is rejected.
You would then need to amend the Cleanup intervals for known/unknown Endpoints (depending on whether you make them known or not) to 1 day so that they are cleared overnight allowing the same MAC to create an account the next day.
There is probably a floor to this so let us know if this doesn't fit the model.