It'd work, but the speed wouldn't be anything very impressive - even disregarding the bottleneck of the card reader's USB connection internally, even the fastest SDHC cards are around SanDisk's 95MB/s, versus a "real" SSD, where you're looking at anything from 300MB/s upwards. (The late 2013 rMBP's PCIe-connected SSD can sustain around 900MB/s - but then, you certainly pay for it)
That said, if it's just a matter of some extra space for occasional use, or booting into a different OS you don't necessarily want using space on the drive, it can work. I've got Ubuntu on such a SanDisk 95MB/s card for just that reason - I'm constantly near capacity on the rMBP's SSD, so having a 32GB card for just that occasional purpose made sense.