In the AGOL settings, you can choose the basemap gallery for your maps. This gallery applies both for 2D maps and 3D scenes.
AGOL manages basemaps in local coordinate systems quite well, including with custom tiling schemes.
However AGOL rejects such basemaps for 3D scenes.
Therefore, we defend that it is indispensable that :
- either AGOL should manage basemaps for 3D scenes in local coordinate systems (not in WGS84) with custom tiling scheme
- or AGOL should provide a separate setting for administrators to define a separate basemap gallery for 3D scenes. We view such a setting as a separate thumbnail under the "Map" thumbnail (see first screencapture in post).
Besides, the specification for how AGOL handles imagery in 3D scenes is only mentioned in an article titled Create scenes/Choose global or local Choose global or local—ArcGIS Online Help | ArcGIS. In our opinion, the limitations in the management of basemaps for 3D scenes should be mentioned in a page dedicated to the configuration of AGOL basemaps.