Is there a way to mod and or change the interface? From the main menu (New Game, Load Game, etc...) to the in game interface?

to some extent yes. most of it is hard coded.
you will need DLTCEP look to edit chu files

HI, mike-db, I´ve been doing this a long time ago for a mod named "Paladins Of Faerun", you can find it on www.sigil.cz on their forums. There is an english section of the mod, too. Take a look.

In short, you have to make your own graphics, if you want a custom GUI, and then you just adapt the original CHU files via DLTCEP (a tool for working with Infinity Engine files). You must also make your own animation files like buttons, or you can use original ones from the game itself.