Flash cartridges for the Atari Jaguar? That would be something I'd be interested in collaborating on. I missed out on all three runs of the Skunkboard. They weren't amazing either, you had to download the game from a PC to the cart each time.
One guy has developed flash cartridges for just about every system going, A guy called Krikzz. He's a very smart bloke. Quite talkative.http://krikzz.com/
Ah, yes, the everdrive. I have heard of that.
The Jaguar is one system I was thinking of, and I know there is demand for.
However what I was looking at was a GX4000 / CPC Plus. There weren't many carts produced for the GX4000, and they are very hard to come by, so a flash cart would be ideal and makes a good starting project for flash carts due to the 8 bit bus and small cart size.
Carts are protected by something called the ACID chip, which has been reverse engineered and the VHDL is out there to replace it. The issue I had was the pinout required for the CPLD to be usefully routed caused the number of macrocells to overflow. Could go up to a larger device, but then it starts getting expensive. The device otherwise is simple -- 3.3v PIC interfaced to a micro SD card over SPI and some SRAM. Bus arbitration handled by the CPLD putting the Z80 into a pause state (jump to self) so the PIC can then program the SRAM, otherwise it reads and latches the value from SRAM.
If a similar xillinx part is more flexible on the pin assignments, I quite fancy trying that...