Servers can send custom models so no client modification needed there. My main concern with attempting to develop any kind of fps mechanics is that the delay based netcode of classicube makes it kind of unsatisfying and difficult to play. At least for any sort of hitscan-type weapons. No custom sounds or camera feedback(shake, first person fire anim, etc) will also leave the gamefeel lacking.
Oh yeah, alecdent mentioned this as well. Definitely doable if someone has the motivation to add it