busybox-syslog postinstall should be able to run at do_rootfs time.
Patch sent on the mailing list.
http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=e02b31623dac54f82509fcb5588bb58daaaa75e0
Verified on master branch. Commit ID: 9ffee9add2d0380f4025e7a678a166f25a7d659e