Support ability to overlay a custom basemap with an imagery service

[mikejeffe]

When you add a custom basemap, typically this image is relatively small compared to the conflation region that you are working in. When a custom basemap is added, only that tile is viewable and everything else is black except for the Locator Overlay which gives you some administrative boundaries and place names with a black background. Would it be feasible to add the custom basemap as an overlay so that it can be displayed on top of an existing service layer?

[mikejeffe]

@sisskind posing the question here to see if there are ideas.

[bwitham]