ok; I have this starting to work (needs a bit of cleanup but what doesn't?)
I ended up NOT making it a plugin because there's all kinds of complications with threading and non-thread-transferable address book objects (and needing a UI thread to ask for permission to access it) that phonegap handles and I didn't want to duplicate.
So I've added a nonstandard field, "ios_socialProfiles". My plan is to make it NOT a default field so "*" won't include it, but if you request it by name it'll bring back the data for you. Seems the best I can do to add an iOS specific field in a reasonable time.
I'll post a patch once I've got it "good enough for my own purposes", not sure if it'll be accepted into phonegap proper but who knows it might be useful to someone :)