Out of curiosity, is there anything in your hack that's specific to this translation or any reason not to release it as a standalone hack for the original game?
No, but since the original game uses kana, there's no reason to change the password system. In the translation, it's difficult to have 64 reconizable characters we can unambigiously write down.
+1 for this; even without switching from 20 to 24 symbols, tweaking the password screen to use A-Z, a-z, 0-9, and 2 other characters, maybe ? and !, instead of icons would be a nice improvement.
That's in the realm of possible but the original games encode characters in two "chunks" of linarly contigious ranges. With the symbols you mention, using the tiles as in the translation, it would require 4 "chunks", 0-9, A--r, s-z and a 4th chunk for the last 2 characters unless you accpet "." and "," as the special characters. Increasing the amount of "chunk" from 2 to 4 would require extra assembly code, so extra hacking effort.
The hack I'm working on keep the 2 "chunks", one being A-Z and the other 4-9. I might reconsider the characters.
That's not even mentionning, there's many confusion from mixed case passwords, I always hated mixed-case passwords and I prefer having a single case password even if it's longer. Mixed-case is OK for unambibious cases, such as A-a, D-d, Q-q, H-h, etc... But it's evil for ambigious cases such as C-c, K-k, V-v, W-w, Z-z, etc... Those letters just look the same in upper and lower case, especially when writing them down by hand.