2013 iMac + Bootcamp + Windows 8.1 = BSOD
I found the culprit. It seems the Apple HFS driver isn't compatible with Windows 8.1. I booted a rescue disk and removed the AppleHFS.sys and AppleMNT.sys files from C:\Windows\System32\drivers and rebooted, and Windows 8 booted fine. 

I obviously lose the ability to have read-only access to my Apple filesystems, but this is not a big deal to me because since I work on a lot of Windows boxes anyway I use external ExFAT-formatted SSDs and flash drives to transport data around.

For others, if having some sort of HFS access was a deal-breaker, there are other solutions out there that can mount HFS partitions. Can't think of any, especially that are free, but in any case, at least Windows now boots on this machine and works as expected.