It is indeed slang for sleep, kip + -age = kippage. Adding -age to a concrete noun to make an abstract is a common contemporary, informal, mildly humerous *usage* (not an example, but the model from which it is derived): Chattage from chat, beerage from beer, etc.
Some quotes: "I have some small coxes in my drawer, and that will have to suffice on the snackage front." "I may indulge in a premature celebratory beerage"; "Last night: home, brief chattage with herself, monster gymmage, home again, snacks, reading, sleep." All from the (UK) Guardian's talk boards (http://talk.guardian.co.uk
), a fantastic source of such things.