Ever played Counter Strike? If not, then I'll explain - when you connect to the server (any server), you automatically download models/scripts/sounds/map/textures that are set on the hosting computer. Say, I made a cool middle-ages-war map, and I made a modpack that matches it, and I ran a server with that map. And I want other people to have those models of mine on while they're playin' my map, so they can "feel the game" or somethin'. So, when those ppl connect to my server, they download not only the temp map file, but also temp models (of mine). After a player's done playin' on my server and disconnects the models that he downloaded are being replaced by his def models (models that he had before he connected). How's the idea, huh? Of course, that would make the loading longer, but it will totally worth it.
And I also thought of models that would be bounded to maps, 'n' stuff... And the beautifulest thing of all is that it's pretty easy to code (in fact, we can even take the SC from the CounterS).
What'd ya think?