So far I've been trying a few things with some random UEFI-related facts found from google.

I have changed the disk mapping to SATA, so

 'qm set 100 -sata0 /dev/disk/...../'

instead of SCSI0.

I have also attempted to map the existing EFI partition with

'qm set 100 -efidisk0 /dev/disk/....-part1,format=raw'

I've been through a couple of revisions to get to the adding RAW as I worked through various launch errors in the proxmox logs. Once I specified format=raw (chosen because it's a real partition on a physical disk) the launch error is now "

kvm: combined size of system firmware exceeds 8388608 bytes

TASK ERROR: start failed: QEMU exited with code 1"