A thought occurred to me the other day about 256k Superbanking. Adding a common TTL chips, a 74174 (hex flip flop), to the 0840 design could extend it to 512k!
The idea is simple - the 0840 board uses VCS-A6 to select banks, except connect its EPROM A12 to EPROM A18. Then feed its RC into the 74174's clock, connect VCS A0-A5 to D0-D5 of the 74174, and Q0-Q5 to EPROM A12-A17. Then you have 512k with two TTL chips.
This might also work as a cheap 127-game menu multicart, but this is assuming you can start the cart in bank 0 with a low on /MR on the 74174. (Or maybe bank 0 or bank 64 if we can't control which bank the 74153 starts in, leaving us with only 126 games ) Might require another RC or something, and these could be unpopulated or replaced by a jumper for use as a 512k cart.
One disadvantage to this is there would probably be no way to have an EEPROM as in the original Superbank board, at least not that I can think of. That, and you couldn't make a multicart that disabled banking once the multicart selected a game as could be done with a PLD program, which means that this multicart would not work with all 4k games. There are also a few more pins to solder. But this all might be worth it to not have to program a PLD, and would allow more people to make menu multicarts for personal use.