MacOS CI for ROS packages would be a great addition, even for core repositories like ros_comm (see e.g. https://github.com/ros/ros_comm/issues/1357#issuecomment-378025569).
You might want to have a look at https://github.com/ros-industrial/industrial_ci, which can be easily added to other repositories without submodules, just loke @tfoote mentioned. Maybe it even makes sense to integrate mac support there, since this is already in use by many packages it seems.