A general facility for promoting selected LOG messages to NOTICE would be nice, but I don't know how to design or implement that. This is much easier, and I find it quite useful.
IMHO that's what we need and it will benefit many more users instead of adding a new GUC every time.
FWIW I recently wrote a patch for Postgres-XL to do exactly this and I found it very useful, especially while debugging race conditions and problems with ongoing sessions. Sorry, I don't mean to hijack this thread, will post that patch as a separate thread.