I certainly support this feature request (which has been partly implemented), but I think it is linked to another issue: the distinction that Roon makes between library tracks and non-library tracks. I can really dig a track, put it in a playlist (or ten), play it a hundred times… but that does not mean it is “in my library”. So, it will not turn up in Focus, no matter whether focus uses boolean logic or not.
FYI: there seems to be a plan to solve this issue “at some point”: How do people typically mark artists/albums for future exploration? - #22 by brian