I would prefer to not maintain different kernel configurations (for
Guix and non-Guix.)

If the initial ramdisk doesn't have enough stuff present to be able to
boot successfully, this seems to provide a good example of entire
directories of stuff to include:

http://www.linuxfromscratch.org/blfs/view/cvs/postlfs/initramfs.html

Even if it might make it bigger than manually tuning it this is
probably easier than figuring out which specific things need copying
and makes it bootable on a wide variety of computers with many
different setups. (And plus I hope for full disk encryption with LVM
some day.)  :)