I saw this on his PBS show this afternoon, but I can't find the recipe on his website. A Google search found a Rick Bayless Mango upside down cake, but it is a completely different recipe.
The one on the show has a muffin-like batter with whole wheat and all purpose flour, baking soda and powder, salt, sugar and buttermilk.
It look like he did it in a cast iron skillet, but other Google searches insist that you should not use iron, because it discolors the mango.
I'm so tempted that I might wing it with the recipe, but I'd feel more comfortable with Rick's measurements.