I see your point. We've filed a Bugzilla entry describing an extension of the single/multiple distinction that should support more types; see http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=2919 for details. You'll have to find a compromise in the meantime, e.g. by taking a note onto the end of the Abstract or similar.
Yikes! Another standard feature of OCS 1 that didn't make it into version 2. How can I explain this to my clients?
I would appreciate a summary/prediction of how this will be addressed in version 2.1, so that whatever customization I do to accommodate my clients' needs will be compatible with the new version.