It's a pity that
@rockbottom hasn't appeared on this forum for a long time.
The project is very interesting and useful. Even in V3 there are problems with games and I really want to re-flash this cart.
I currently began to reverse and study the V3 boards.
It is almost like the previous, but differs with 1.27mm headers for stilts.
I developed three boards for my STM32 dev board for programming P, S_M and C_V chips.
I'll keep you informed.
Another (crazy) approach is to program the cart boards right in the slot. To do this, we need two 120-pin slots and a MCU.
Unfortunately, some pins (WP#, etc.) are hard-wired in the cart's PCB, so I decided to start with separate boards first.