Could you get a 32GB SD Card for the mini and use it to boot from? Not sure on speeds etc. and how good it would be (sorry I haven't searched this time)
I think you could do it. The SD card reader is just an internally connected USB. Speeds would be atrocious though. Good way to have a bootable backup image though
My mid 2010 Mac Mini boots into Snow Leopard from a 16G Class 10 SD card with decent speed, at least faster the stock 5400rpm HD, but slower than a swapped in 240G SSD.