's suggestion about extending the protocol has got me thinking. He isn't the first person to ask for a way to determine default userpic URL, and I'm sure plenty of clients could make use of such a feature.
Another common client problem is using getting events from a community journal. The getevents mode does not indicate the post's author, so the client has no idea if the logged in user is allowed to edit the post or not. This could easily be remedied by updating the server code.
It seems to me that no client authors are involved with server development, and the server developers don't seem to give priority to feature requests by the client authors. I'm not pointing fingers here; I just think we have two separate communities that don't seem to be communicating.
Thus, I think we need somebody to compile a list of protocol extensions and start implementing them, and become the Client Ambassador to the Server Developers. Either somebody who knows the server code and wants to win the love and admiration of the client developers, or a rogue client developer can learn the server code and we can add them ourselves.
I've been thinking of taking this on myself, but an opportunity (schedule-wise) has not presented itself. Besides, it's so much more satisfying to delegate. :-)