okay, there have been many discussions in the #alientrap channel how to solve this
In v1.6 there will be three additional characters. These characters can't be chosen in the options menu. However, a server can set these characters as default-characters (e.g. on ladder servers) so that all players will get that character
The three characters I'm talking about are modified versions of the "Mulder" character, which is based on the "Nexus" skin. I chose that model because it's one of the biggest. Here you can see screenshot of the new characters:
The red one
The green one
The orange one
I also created aliases which could be votable on the server. e.g. "forcefullbright_green" (or ...red/orange) will set the default player model "Nexus" and the "skin" to the green skin. All players on that server will now use the green model. "forcefullbright_off" can disable that, ofcourse
There are also aliases for the player himself to chose a certain fullbrightskin. This could be useful if player A wants Player B to be yellow, while Player B wants Player A to red. Player A then enters "takefullbright_red" and Player B enters "takefullbright_yellow" .. and everyone is satisfied.
However, this is just the technical aspect for now, it is doable and these fullbrightskins are supposed to be included into v1.6. This would solve the problem of violating the rules because of hacking exisiting files. Of course there should be a defined rule setup for the handling of fullbrights, e.g. a rule that says that fullbrights are only allowed if both players agree, or "the challenged player choses whether to use fullbrights or not", etc.