@zestyping worked his 
magic and Collect v1.23.0-beta.3 has a beautiful new Maps setting screen with base layer source and reference layer sections. The reference layer section shows the offline layers from the device. To add reference layers to the device, see the documentation (will be updated with new screenshots soon).
If Mapbox is selected as the base layer source, MBTiles files containing vector tiles are supported. These will be displayed as colored outlines for now and styling will come in the future. Please join the beta (ODK Collect v1.23 Beta), try it out and provide feedback!