I was thinking of something like this would be nice too. I originally thought that the server could just push its config on connect, but then I realized that most likely all the mods are already configured by the time you're done with the Mojang logo. So the other option would be a launcher/client mod/server mod combo.
The server mod would have its mod config files pre-packaged as a zip (probably created on server start) with a checksum, and when a new client connects, the client mod will check if it has the server's file, and if it doesn't, download it. The client mod would place this zip in a location and then set up some sort of path reference for that config. Then you'd be asked to restart minecraft. By default the client mod would have a config that stated which of those config downloads was active based off the last connected server, but would easily be able to be changed by a launcher. Really, they wouldn't have to be separate mods, just different portions of the same. And since the server paradigm is also there for single player play, it would by default create these config setups based off the local config files for Single Player/Lan games.
The important thing would be if the launcher could integrate selecting between these configpacks before start, to prevent having to do too many game restarts to fix configs.
I'm even somewhat interested in seeing what it'd take to actually do the mod work myself, if someone had some good starting points.
