In the 19.1.0 beta which we released last week yes we support the side loading of a TPK and their use as a basemap behind offline areas you downloaded onto the device from your webmap. Is this the functionality you are referring to Marty?
Hi Brent. I am running the latest explorer beta (19.1.0 (2602) ) and am able to view our offline areas that we generated. I do not, however, see the option to select basemaps from the overflow menu when viewing an offline area (or anywhere). The only options I see are Bookmarks, Legend, Measure, Share Map.
I have placed our TPK's in a folder named Basemaps and into the explorer root (just like we do for collector, where this works)
Am i missing something?