People that need to have fine-grained control over FEATURES=buildpkg would benefit from having a package.buildpkg configuration file to express the information.
I like this idea as well, but to be clear, it's fine grained control over the packages I don't want binpkgs for which is lacking. Right now I add a list of things to EMERGE_DEFAULT_OPTS to exclude binpkgs from being built or used, it would be great if that was a file where I could exclude things.
One possibility could perhaps be to patch --buildpkg-exclude to accept package sets as well as individual atoms?