Can you do this?
With Windows? No. Technically, it's possible to boot an OS (like Linux for FreeBSD) off a SD card - you simply point your bootloader to the bootable partition of the SDXC card and away you go But...
Why would you want to do this? It would be horribly slow.
(and by slow, I mean slow as molasses in winter slow)
Microsoft doesn't like people booting from removable drives and since the SD card reader is off your USB bus, it shows up as a removable device. Linux doesn't have this limitation, however.
This doesn't, however, prevent you from putting a virtual HDD on the SD card and booting Windows in a virtualized environment like Parallels or VirtualBox.
Why you don't want to do this
Your SATA drive on your Mac transfers at 6Gb/s. A USB 3.0 drive goes 5Gb/s. Your SD card (assuming class 10) will go at most .5Gb/s (yes, only half a gigabit per second).
*Basically, you will be trying to install Windows on a disk that's 12x slower than the drive built in to your Mac. And that's at the highest speed possible that an SD card can go.
I give more detail in an answer here
By trying to boot off the SDXC card, you would bring your MBP down to a crawl; to the point it would be torturous to us. The best thing to do is to keep that SDXC card for pictures, or whatever file storage you need.