Currently, getmaskingreason only searches profile package.mask files. It can be extended to also search /etc/portage/package.mask.
Actually, it already searchs USER_CONFIG_PATH. It was updated to prioritize messages from USER_CONFIG_PATH in this commit: https://gitweb.gentoo.org/proj/portage.git/commit/?id=cc3ca52718e0f15a2d0a3f3360977e3b33041db9