Update, January 2024: boydsmaps no longer has maps from Mapbox, so the original links here don't work anymore. I have replaced the two links below with similar maps that are locally hosted at boydsmaps.
Just playing around this afternoon I had an "aha" moment, wrote some quick code and we now have 3d terrain and sky fully implemented in the boydsmaps app! Like they say, "it's easy when you know how".
https://boydsmaps.com/#12.53/37.741809/-119.592947/mbx3dnalc/57.60/71.60
Or closer to home...
https://boydsmaps.com/#18.84/39.847823/-74.286762/mbx3dortho/-80.40/80.00
Now, the only thing I couldn't get working was changing the vertical exaggeration "on the fly". So I have created three separate maps - one with no exaggeration, another with 2x and one more with 3x. This actually seems to work very well, just flip between them to see the effect.
When you choose one of these terrain maps, you'll be able to tilt the view much farther than other maps which allows you to see the sky. If you switch back to a conventional map, the tilt will snap back to its usual limit.
Now, this is brand new code with minimal testing. So check it out and see if you have any problems. These maps don't perform quite as well as they did in my 3d demo example, due to all the overhead of the boydsmaps app. They seem to work acceptably in a "normal" sized window but are slower in a large full screen view. They may be a little more than older phones can handle, have not done any testing on a phone yet.
As with other updates, it may be necessary to clear cache, delete site data or even delete the app from your phone and re-install. See the discussion of these issues above. And again, the Mapbox maps aren't free, so I'll be monitoring usage to see if usage exceeds the allowed quota of free imagery and may have to pull them if it's going to cost me $$$.
Another thing to consider, if you are using waypoints they will work fine on these maps however the locations will be deceptive. The waypoint location will be at 0-foot altitude which will not appear to be correct if they are on a mountain.