Thanks to everyone who responded both on and offlist; the suggestions were very helpful! As such, here is a updated version of the Pac-Man Dossier with better page graphics, a much-improved introduction, and a new chapter devoted to the "split screen" level (including a map of the hidden dots). As always, I encourage people to send in their feedback, corrections, and suggestions on the guide.
From what I understand, the kill screen isn't a result of the level counter itself overflowing into other areas of RAM. Instead, it's a result of two factors:
-1- The 'show fruit' routine works by showing a fruit, decrementing a counter, and seeing if the result is zero. If not, it loops and keeps drawing fruits until the result is zero. Thus, an attempt to draw zero fruits will draw 256.
-2- Since the game would malfunction if it tried to draw dozens of fruits, the game limits the number of fruits drawn if it's greater than seven.
The kill screen occurs because the game decides it's save to draw zero fruits (since zero is less than seven), but it actually ends up drawing 256.
BTW, on Ms. Pac Man, I once saw a copyright notice appear near the bottom the screen, obliterating some of the dots. The player and ghosts were able to pass over the copyright notice without impediment, but there were no dots there. After the player died, the copyright notice disappeared but the board was still short of dots and so the game was unwinnable. I think the player was on the second level--it certainly wasn't a level-256 killscreen, but it was an anomaly I'd never seen before or since. Has anyone else ever seen such a thing?