Will - I agree, thinking outside the box is absolutely a great idea. Just remember these 3 things when playing around with new submissions.
Ask yourself these questions IN THIS ORDER:
1 - Do I have control of my opponents body?
2 - Do I have control of the body part I'm attacking? (Arm, neck, foot, etc..)
3 - Do I have a way to apply pressure?
If you answer yes to these 3 questions ....then play with that submission.
In the case of the Arm Triangle from bottom mount.....You do not have control over your opponents body from the bottom mount.
I hope this helps in the future......I teach all of my students these 3 fundamental rules to applying submissions.