What is the "options" section in the block diagram? Does that represent the debug functionality you mention on unexposed pins? Anything else interesting that someone could do with these chips in a different package?
This makes me wonder if you put one of the other test pins on a scope, if you'd see a pattern representing the song or sequence, allowing quick verification of the chip. I think at 512x speed, even a more limited storage scope of the era of these chips could capture the full output on-screen. Each flavor of the chip probably had an expected pattern based on ROM which could be checked.
I'm imagining a technician sitting at a test bench, having to do verification work on these all day, thankful for having a test pin instead of having to listen to the same dang beeping song over and over :)