I found this out by accident just searching other forums, and realized it’s something a lot of people would take advantage of if it was more widely known.
Guild Wars supports use of custom music, all you have to do is create a a playlist and place it along with the music files it contains, into your Documents/Guild Wars 2/Music folder. As of right now the game supports 8 different playlists, named:
Ambient
Battle
BossBattle
City
Crafting
MainMenu
NightTime
Underwater
Most standard music formats seem to work, although i’ve noticed that .m4a (AAC) doesn’t. Any missing playlist won’t cause an issue, Ambient will just be played instead.
Also it plays off the list at random, not sequentially.