I did few test.
Windows rt install.wim unpacked to SD without integrating any drivers: error INACCESSABLE_BOOT_DEVICE.
Windows rt boot.wim as winpe from SD without integrating any drivers: boot OK (thouch didn't work).
Windows phone 8 ffu converted to vhd and copied all files(include data,efiesp) to sd(one partition) without editing registry: boot failed (no boot logo, no bsod, just black screen)
have you done much with openBLT and EFIDroid?