This is mostly a shot in the dark, but is it feasible to have a /boot partition with a initramfs on a partition of it's own and then all the rest of / on a raid partition?
To make it work kernel, initramfs, mdadm, GRUB and some other tools would need to be copied to the /boot partition from store, but this is potentially worthwhile? (My motivation is that I've a machine with raid "inside" and guix system on a 15G usb flash disk "outside". Lately it keeps running out of disk space when I try out more interesting setups like http and similar) -- :)