Depends on the mod.
If it's only scripts, no.
If it's textures/shapes terrains... Yes you'll need to download something to join the server (Not automated)
He's most likely trying to do a "scope" around the reticle.
Something like this:
http://images1.wikia.nocookie.net/__cb20100406044037/combatarms/images/2/29/MG36_Scoped.jpg
And yes, very doable.
And no, I won't be doing this.
Frankly, I'm not the one to mess about with the engine code to get the things changed so that we can let you host dedicated servers. And thus, custom loadouts is what you'll get from me next.