Hi Adam,
I think something like this is close:
var imageryProvider = new Cesium.TileMapServiceImageryProvider({
tilingScheme : new Cesium.WebMercatorTilingScheme()
});
But it likely won't work because MapBox requires the access_token query parameter, which the TMS provider does not support.
It should be really straightforward to fork TileMapServiceImageryProvider.js and make the necessary modifications, though. Better yet, a generic "template URL" imagery provider like Leaflet's TileLayer (
http://leafletjs.com/reference.html#tilelayer) would be easy to create, and we'd welcome a pull request if someone implements it.
Kevin