Well.. what you just described just screams for extension mobility.. set the default user profile to something very restricted via css/partition so the phone can only be used to call the reception. Then if logged in, people have the right to make outgoing calls, etc. Combine that with automatic phone logout and I think you have your solution, all without having to write a single line of code.
As for changing names.. there are different considerations. The users in ccm are stored in the ldap indeed.. if you know ldap, you can make modifications directly on the dc directory. Otherwise there's AXL (guides for AXL and everything ccm related are here: http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guides_list.html)
Now, that changes what you see in the phonebook. If you want to change what's on the line, you have to change the line.. alertingname is the keyword. AXL once again comes in handy. Or, there's SQL.. the field is in the NumPlan table. On CCM 4.1 you can just read it via the AXL command sqlexecute but you can always access at the sql db (it's dangerous though) via odcb or whatnot, and starting with ccm 5.0, there's a new axl command sqlupdate which allows you to send sql updates to the ccm db.
