Ok, great information in this thread! Thanks all.
So, it sounds like we can summarize this as follows:
1) The "Relationship Calculator" Tool gives the same information as the status line. It is quick, and doesn't show in-law relationships.
2) The Quick View "Relation to Home Person" finds the closest common relative between two people, does take into account some in-laws, and is still very quick.
3) The Family Lines show some of these connections, but not all.
4) The Disconnected Person Filter will show you only people that aren't connected to anyone else... it doesn't show people that aren't connected to an arbitrary person
So, it turns out that I wasn't really interested in the relationship between two people, I was more interested in how/if they were connected. I also didn't mind if GRAMPS wanted to chug through the data for awhile, as it would save me time from doing it by hand. So, it sounds like a new use. I'm not convinced that we need yet another variation, but I've added an experimental, slow, thorough gramplet called, for the moment, Deep Connections. You can see a picture here:
Perhaps this should be a 3rd-party plugin? Let me know if you find this useful, and different enough to include in GRAMPS.