Can Hyper-V R2 be installed on an SD card for use in servers that support booting from internal SD (i.e. some HP Proliant and Dell)? All I can come up with online is that it should be supported as of R2, but nothing specific as to whether it's actually supported now that R2 is out.
If so, what kind of SD card is recommended?
Hyper-V can boot from anything Windows can boot from. So, yes, Hyper-V can boot from SD, if the firmware can do it.
But many people who are interested in booting from one type of flash or another are assuming that you don't want to do any write operations to that flash. Hyper-V does not have any provision to make the flash read-only or to send writes to some other media. These things exist from third parties and they should thoretically work with Hyper-V, though I haven't personally tried them.