The one dll I create should work across all mods, all it needs to do is adjust some paths.
ie. from "models/hostage.mdl"
to "models/cs_office/hostage.mdl"
It doesn't get any easier than creating a directory with the same name of the map in your models directory.
When the model is being precached, I'll tell it to look for it in models/mapname/ first. If that doesn't work ( pfnPrecacheModel returns <= 0 ) then it will use the default model instead.
No config files needed
