One thing that would get me interested back in NTT modding (and which was one of the first reasons i started doing it) had a little bit to do with adding options, modifying UI and menus (specially the main menu), but since the only way to load weapon is ingame, i guess it's not possible??

If that's the case, is it possible to add it in? it would open possibilities for many different mods, but i understand if it's too hard to do something like it

It's not something that would be "too hard," it can easily break the menu systems that Vlambeer hastily slapped together with some YAL magic strapped onto it, and multiple UI mods would pretty much break everything. You can use commands if you wish to make your mod configurable, check 9joao6's PVP mod for an example.

Main menu is outside of the game state - no mod-related code runs at that time, replays cannot be recorded, and generally things work... differently. I have briefly evaluated doing this when starting to integrate modding support, but it could have too many side effects to make it a worthwhile development. You can totally replace the character selection with custom menu(s) and set that up to load via startup.txt though.