Unfortunately I don't think you will be able to do that as the phone is setup with a certain firmware.
However, with the SNOM 300 phones being listed as compatible with OCS now, you could look at a solution like that.
The snom phones can register under multiple identities, so you could have it registered to OCS, and then have another registration to a SIP paging system, so it would act as an endpoint.
Go to http://www.snom.com/ and poke around, I don't know if any of their devices will actually register with a SIP paging system, but figured its worth a shot.
Randy Wintle |MVP:Communications Server, MCTS: UC Voice Specialization | Winxnet Inc