Seriously cool!
BTW - if you or someone else need to do this again, here's a board a friend of mine made that uses the 144 pin STM32F407.
It has the pads for an SD card reader (connected to the SDIO peripheral - tested sustained reads at 10 mb/s!), USB, and an SRAM chip. You could just not put in the SRAM and have quite a few more GPIO pins, it's all broken out on pinheaders. As an added bonus, unlike the Discovery boards, the GPIOs are laid out in order here, so no hunting around to connect consecutive GPIOs!
There's no MIOS port, but it's basically the same as the Discovery just with the bigger chip and without the external audio stuff, should be easy to adapt. We had a couple made and assembled them by hand and I've used it for a few non-permanent things, and it works fine.
Kicad files here:
https://github.com/knielsen/pcb_stm32f4_sram