I just came across this photo, where someone used ribbon cables to relocate the Fast ATA 1200.

Usually there's ton of dead space towards the front of an A1200 tower with a Mediator. To me, the obvious solution would be to design a PCB that relocates the Fast ATA and ROMs from under the Mediator and into that dead space. Turning the ROM sockets though 180° would minimize the size requirements of such a board.