Anyway, I completely redesigned the RAM cartridge, this time using a correctly sized edge connector, correct pin assignment, and a through-hole EEPROM.
Furthermore, I tried to stay within the confines of the original cartridge board area. The packing is dense but not too crowded. Soldering the components was quick and painless. The board is suitable for both 64kbits (suitable for 2 DX7 voice banks) and 256kbits (8 banks) EEPROMs. To test the board I use the simpler 64kbit device and a an ordinary sliding switch in the position of the rotary coding switch (top left corner of the board), and only one additional resistor. I didn't include a pad for the 2.2uF polar capacitor, but managed to mount it between 5V and ground by soldering it to the power leg of the EEPROM and the bottom pad of R1 to the left of the memory chip. The finished board looked quite neat and tidy, a far cry from the mess of the previous prototypes. I couldn't wait to test it in my DX7.
Well, make that last one "I couldn't
This is very unfortunate, because I like the quality, price and ease of use of OSH Park, but for this project I can't use their service. I need to find a different prototyping service that allows specifying thinner boards, perhaps 1.4mm or so.