I did say I was probably wrong, but that's what I would have tried to do first.
This is what I meant (basically a hypothesis):
Let's assume the meteor attack is is a separate piece of code all together. Everything required for the attack (graphics, AoE, damage) should be in this paragraph of code, and should execute when Xabaras's AI tells it to. Now, we copy that piece of code and tell it to execute when a player presses X + C while the skill is equipped (as I can see, you have already figured out how to make new guns / skills). Now all you need to do is change the 'spawn point' of the attack to in front of the player (if there is a spawn code) and change the damage.
It's all just an idea which is far easier said than done, but it's one way you could try IF everything went to plan (like there's a code for the meteor attack w/ spawn point and no reverse engineering of bout.exe needed - which would be the hardest to face if it was).
Ah well, I hope we'll figure it out one day!