As I understand it, each mod would have to be updated to be compatible with have the snapshot. If the mod is not updated, you would have to disable it for the server. As each mod is updated to the snapshot, the server and their users would have to update. You could find each mod over on the minecraft forums, follow their updates, and update as they do. These snapshots are, in part, designed to give modmakers time to update their mods quickly after official releases.....
While a vanilla server could do this easily enough, heavily moded servers would require almost full time server administrators. FTB allows for non server administrators to have a server that is up to date with current offical MC releases within a week or two of those releases. Anything more than that is a DIY project.
