There is a workaround, essentially a script that manually copies over the relevant files into an unencrypted /boot:
https://issues.guix.gnu.org/48172#4
If you wanted to preserve rollbacks, you'd have to copy all the relevant generations boot files (kernel, initrd, dtb) and adjust the extlinux.conf appropriately, and plan for a larger than expected size for /boot... because this is #Guix :)
Unencrypted rootfs blocks me from really using it more, too!