Good work on the puzzle, Dalek and others. The 26 unique characters seem to indicate that there's an alphabetic character correlation for each yellow block. But no luck at all decoding a message with frequency analysis, reading the map in many directions (including spirals).  I have a slightly different distribution for your 1b, 7a, 6b, etc; just labeled it differently (and I spent hours painstakingly reading the map): 12 11 10 10 10 8 8 8 8 8 8 7 7 7 6 6 6 6 5 5 5 4 4 4 4 4. That difference isn't really significant for frequency analysis though.  

After so many tries, I’m wondering if this just doesn’t require frequency analysis after all. Yet after trying other strategies for assigning alphabetic characters to the yellow blocks, I’m just stuck.

In the past, Coin_artist's art puzzles presented a relatively easy-(but not simple)-to-find bitcoin address, and the ensuing stages of the puzzle eventually led to finding the private key. I’ve got to assume that the current step should be relatively easy. It’s a rabbit hole and is supposed to entice us. And it shouldn't require advanced programming or crypto skills. I don’t have those, yet I have been able to keep up with most stages of her prior puzzles.

I do believe, though, that the keypad entry for the text adventure will be discovered by interpreting the map somehow.

Those are my completely unhelpful thoughts.