I am happy to report that the mystery of Mr. Runch has been solved. He is an intergalactic criminal from M&M’s Break ‘Em, a puzzle game for the DS. That still doesn’t explain why they called that specific event in M&M Beach Party was called Mr. Runch Slam, but I’m ready to say ‘case closed’ and move on with my life.
I had assumed playing three M&M’s videogames would cover everything we need to know about the lore of the M&M’s world, but apparently we needed to play all seven. Not to make excuses, but in our defense M&M’s Break ‘Em doesn’t even make Wikipedia’s list of M&M videogames.
Hat tip to @LauraWatson for letting me know, and additional thanks to SonicGamer 360 for uploading 10 minutes of M&M Break ‘Em footage to YouTube.