That would be easy to solve, i guess it's the main reason they went with Steam: each server can decide what kind of mods/3rd party content to allow, even if you have that 700km/h 109 you won't see it in the menus plus you won't need to have multiple installations for each mod pack.
I'm not very fond of having to use steam but i'm not going to discredit its good points either, running a list of approved mods from a pool of many without needing separate installs for each individual server is one of its strong points.