I've been looking at this for some time and basically was able to put together a workaround:
-Switch off UEFI boot and VT-d
-Install ESXi 5.5U2 to a USB stick (You might have to actually press F10 to boot - just choose the stick without UEFI prefixed)
-Download the latest patches from https://my.vmware.com/group/vmware/patch#search
-Save them to a datastore
-Use whatever patching method and install ESXi550-201410001
-Reboot to BIOS
-Turn on VT-d and reboot
-Again you might have to press F10 to pick the USB stick
This will allow 5.5U2 with VT-d passthrough to function correctly. I have not tested UEFI as yet but at least without it, there is still a chance to make this board work.
Of note: I used this with the Intel AMT KVM in action and not using any fancy keyboard or monitor (Never physically touched the box)