Autodownload support in client. On detecting a missing resource, the client requests a URL from the Game Server. The client has (plugin, client side DLL, Java, builtin by id) means to use FTP (and/or HTPP) for download. The Game Server only transfers a string, by default, a separate Resource Server should be used for download.
A minimal dependency of the server on other servers providing the resources. Additional setup for the server admin to provide the URL's. Minimal bandwidth and performance penalty for the Game Server on connect of a "naked" client.
Autodownload is already present in Quakeworld and so should be easy to implement. Pointing the client at an alternate server is only a minor extension. Adding the GUI and the configuration variables to allow the player to prevent or intervene in the download process, including announcement of the individual/total amount of data to download is more involved.
Does not address the problems of getting the skins on the server (maybe an autouploader for that).
The community could set up the entire support structure (see ModSpy project), given the hooks into, or FTP/HTTP support in, the client.