[Other Question] Overworld music continuing through a battle

[BLΛYZE]

Is it possible to have the current overworld music to not be interrupted by a battle?
If you don't know what exactly I mean, think the final battle against Wally in ORAS where the theme already started before the battle did, and it carried on into the battle.

I've managed to carry the music into the battle by having the same battle music set in the metadata of the map as the overworld music. But because the victory theme is a ME and not a BGM, this doesn't work for the end of the battle... Maybe there's a way to grab the music from the BGM folder instead of the ME folder? Or anythin' like that, I dunno.

 

[BLΛYZE]

I've tried putting that $game_system.setDefaultBGM("battle2.ogg") script command into a trainer event and it still played the victory ME when I defeated him... (I tried battle2.ogg with and without the quotation marks, and with and without the file extension)
I also tried resetting the metadata of the map and it still didn't change anything...
Do I have to use it in a different way?

(EDIT: I actually found out how to achieve exaclty what I wanted by pure accident. I made it work by simply changing the victory ME in the metadata to a file that doesn't exist.)