UEFI display frozen on Retina MacBook Pro

I have a "MacBook Pro Retina, Mid 2012" (MacBookPro10,1) on which I'd like to be able to boot FreeBSD from an external USB drive. For testing I've been using the mini-memstick images from the -CURRENT snapshots, most recently the one from 20140903.

I am able to select "EFI Boot" on the USB device from the Mac's boot menu, and it does _something_, but the screen never changes--the image of the boot menu is displayed indefinitely. I think it is actually booting since there is drive activity and the caps lock key indicator starts working a few seconds in, but the screen just stays the same. Thinking the resolution of the Retina display may have been an issue, I tried booting with it disabled (lid closed) and an external monitor and keyboard. The result was the same--Mac boot menu frozen on the external display.

Is there anything I should try to troubleshoot or debug this issue? Anything else I should include in a PR? I can test patches if needed (probably after building an image including the patch from a VM).

Thanks,

JN