figlfdev donated a laptop (most of one, anyway) to the cause. This one is new enough to have uefi, so I now can test on real hardware. And I have tested. A multiboot-usb that has efi files added from a snapshot that was made with the test version of refractansaphost works. Actually, two different usbs made from two different isos booted with uefi.
fig, that second usb needed /isolinux/isolinux.cfg, and I forgot to add that. It's only used for grub to find the root of the device, so it can be empty, but it must be there.
I'll continue the technical discussion over in the Discuss section after I do some more testing. I just wanted to post this and say "Thanks" to figlfdev. This is a big help.