I'd like to use the basemap gallery widget to allow an easy way to switch basemaps. I have various years of aerials as well as a custom "vector" basemap covering our particular area of interest. I want to be able to switch between the various basemaps with the widget but always show an Esri basemap behind it to cover everything outside our area of interest. Is this possible?
Figured it out! When adding a basemap, you can also add a basemap layer to the basemap.
The order the layers draw depends on the order in which they are added. The last basemap layer added shows up on top and continues goes in order up the list (the first layer added is on the bottom). I got the Esri basemap URLs from http://services.arcgisonline.com/arcgis/rest/services