Here's some new information on the topic:
Using the built-in "Mapbox @ OpenStreetMap" I can zoom to level 21.
However, using a custom map (also hosted on Mapbox) the tiles stop rendering @ zoom level 19. I'm not sure if this is a limitation imposed by Sense, or if Mapbox won't send the tiles.
Has anyone else encountered issues (or a requirement) like this? The 3rd party solutions appear to be limited even more in their zooming abilities.