I also plan to run Guix System on my Reform once it arrives. I don’t have any aarch64 experience though (prhaps I should have gotten an iMX.8 development board or something). I’d be surprised if getting Guix System to work would require anything that didn’t meet the Free Software Distribution Guidelines that Guix follows. I was under the impression that any non-free fimware needed was loaded before control was passed to the user’s operating system. I think we should try to get support in Guix proper as a first choice before resorting to a separate channel.
I do anticipate having to get creative with the following constraint though: I would like to run my system without an SD card and with an encrypted btrfs filesystem on NVMe. My experience on x86_64 hardware is that Guix configuration is more straight forward if the bootloader loads the kernal and initramfs directly from /gnu/store, not a separate /boot filesystem. I don’t know much about uboot. Does it have support of luks and btrfs? In the case where it doesn’t, I think I’d what to chain load grub from uboot, and then use that to boot the system.
Looks like it will be a fun ride 
. I can’t wait!