There is no specification about raided UEFI. That was somehow missed or left out in the specification maybe due to the implementation on hardware RAID side. ZFS consists of software raid and there is simply no solution for UEFI on software raid. This is not a PVE problem per se, it's a general software raid problem. IIRC, the grub people are working on this, because they suffer from the same problem, so does syslinux and any other boot code.