Schematic and rendering uploaded a few minutes ago for my next step in hardware. This will give me two MicroSD cards for persistent mass storage. Normal operation will likely only use a single card (even the smallest of modern cards is more than CP/M can properly make use of). Second card slot, however, is part of my goal to make the Firefly board entirely self-sufficient. BIOS will include the ability to clone cards for backup.
Still a long way to go before I'm ready to tackle CP/M. Aside from coding BIOS support for managing the SD cards I've also got an Interrupt Mode 2 console handler to finish.