Currently, registration is a one-shot deal -- you are registered for the conference or you are not. However, we're talking with a few large users of OCS who are interested in contributing greater flexibility to the registration process, i.e. allowing users to choose whether they want to register and pay for additional items like banquets. If you like, I can put you in touch so that you can discuss specifics and potentially coordinate your efforts.
If you do start developing with OCS, I'd suggest becoming familiar with the current CVS version because a large number of features have been folded into it from OJS 2.2. The CVS version of OCS is currently not stable, but we'll be working on it very shortly. It might be worthwhile looking at least at the changed database structure, as all metadata storage has been overhauled since OCS 2.0.
Public Knowledge Project Team