It should be possible to accomplish this with existing options by suffixing all of pants BUILD files (ie, as BUILD.pants), and then setting the build-ignore option in pants.ini to something that will cause unsuffixed BUILD files to be ignored:

$ ./pants help-advanced | grep -A3 'build-ignore='
--build-ignore=<str> (--build-ignore=<str>) ..., --build-ignore="[<str>, <str>, ...]", --build-ignore="+[<str>, <str>, ...]" (@fromfile value supported) (default: ['.*','/dist/','bower_components','node_modules','*.egg-info'])
    Paths to ignore when identifying BUILD files. This does not affect any other
    filesystem operations. Patterns use the gitignore pattern syntax (https
    ://git-scm.com/docs/gitignore).