Have you used Favorites yet?
You can Favorite all clips that don’t have keywords.
Make a Smart Collection of Favorited clips.
Keyword clips in the Favorite Smart Collection. As you are done with a selection, perhaps after some number of keywords. Un Favorite them, controlling their disappearance from that Smart Collection.
In some respects it’s a variation on Bob’s suggestion but instead of using a “meta” keyword, you’re using Favorite which has a very easy Un Favorite (un rate) option.