I had hoped I could avoid having to dive into WordPress plugin development. Might it be worth a shot to contact some of the devs of the leading plugins that are almost there in functionality? Or perhaps I should hire a freelancer…

I have to wonder, though. This seems such an obvious and useful thing to have, especially with complex non-hierarchical taxonomies. Instead I see usability nightmares like walls of checkboxes, mitigated (made worse) by collapsible sections and such.