[Serusers] Asterisk + SIP integration for SIPLE. Newb needs advice.

Ive administrate several Asterisk servers, and sometimes one company will have multiple servers with their own respective users. Unfortunately users of one server are unable to see the presence state of a user on another server. I understand that SER can (a) be my presence agent or (b) can pass SUBSCRIBErs to a respective asterisk box, but doesnt that result in (a) clients subscribing to SER cannot see the presence states of local channels on asterisk (i.e. a parking spot), or (b) clients not being able to see the presence of clients SUBSCRIBing to a different asterisk server (back to square 1)? Can someone help me wrap my mind around how this could/should work. Perhaps it can't be made to work at all. All i'm looking for is a general direction to research in. The question as best i can phrase it susinctly is "How can I use SER to allow clients of disparate asterisk servers on disparate lans to have a centralized presence roster?"