Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
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
 
Not really so.

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.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.