The file format is easy...

bind 1 ACTIONBUTTON1
bind 2 ACTIONBUTTON2
bind 3 ACTIONBUTTON3
bind 4 ACTIONBUTTON4


This breaks down to a console command, the key to bind, and the action to bind it to... enumerating the binds for different mods like bartender, trinity, etc would be easy enough as well.

This leaves creating an application that would display keyboard and accessory layouts, and let you setup the binds for them based on account/character names. Saving this to a profile would be handy as long as the profile saved account names in a separate file it could be easily shared.