The buildroot documentation mentions an option for a "Post Image Script". A script that will be executed after the images has been created.
I cannot find this option in the newest release. It's supposed to be called "BR2_ROOTFS_POST_IMAGE_SCRIPT"
Documentation is here: http://buildroot.uclibc.org/downloads/manual/manual.html
It seems that the newest release is based on a rather old release of builtroot.
The Post image script is enabled if you download from bfin trunk.