I just had a thought, look at this.

Terrains:: LUSH ARID ROCK_DES TEMPERATE MORDOR

Architecture:: LUSH W_EURO
Architecture:: ARID S_EURO
Architecture:: ROCK_DES ISLAMIC
Architecture:: SAND_DES ISLAMIC
Architecture:: TEMPERATE E_EURO

Game_Types:: SP_BATTLE CUSTOM MP_LAN MP_INET

MP_Styles:: LMS KOTH CTF

(I will add MORDOR architecture before I test it)

Do you reckon I could create a map using a different texture set and then simply edit that part of the file to change it to MORDOR exclusively by deleting all the other texture sets?

EDIT-I tried as you suggested but it simply removed Agincourt as an option.