Boot Camp Assistant had successfully created the Windows install USB key, and I was able to boot into the Windows installer. It was when I tried to format the Windows partition to NTFS did I run into the MBR/GPT issue. I found and installed a OS X command line utility 'gdisk' which changed the hybrid MBR data to allow Windows to install properly. All details are noted here: -detects-gpt-disk-as-mbr-in-efi-boo t