--buildpkg-exclude "<atom>+" should override FEATURES="buildpkg" and -b.
This is fixed in 2.1.10.28 and 2.2.0_alpha68: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=2d78dcda11d753a54f821c7fb482b4bb3b511d0b