Putting this in config/scripts/FAIBASE/50-mkinitrd is all I needed.
Apparently I over-complicate things....
#! /bin/bash
error=0 ; trap "error=$((error|1))" ERR
# create an initrd that will actually boot
TARGETVERSION=$(/bin/ls $target/lib/modules)
mkinitramfs -r $target \
-o $target/boot/initrd.img-${TARGETVERSION} ${TARGETVERSION}
exit $error