I think the best answer is still to use Bookmarks—which does just about everything you could want from this kind of integration, that is feasible. You get a preview in the sidebar if that is what you really want, but if you want to modify or edit this thing (which evidently has no connection to any data in Scrivener and is a complete disconnected data model that has no inherent benefit in being in Scrivener) then you double-click the bookmark instead of single clicking it. Easy---and it works just as well for Excel spreadsheets too, or whatever else you need.
This is brilliant! I did not know really what Bookmarks were for until you mentioned them - Just tried it with a Scapple file and presto - what a great way to access mind maps! Thanks.