After a resolving a few odd and ends I was able to create a proof of concept of the deferred environment probe idea I talked about earlier. The environment probe is basically the same as a point light. They have a range and affect a finite area. The color property can be used to adjust the intensity of the ambient lighting / reflections. Basically, you want to cover your indoor environments with these, but it's okay if you miss some spots. The environment probes fade out gradually so you don't have to be too strict about coverage. Also, if two proves overlap the brightest value will be used, but they will not add their lighting together.

There are still some issues to figure out, like how to control the miplevel that is used in the cubemap lookup and how to automate cubemap rendering in the editor. I'm not going to try to make the cubemaps update in real-time in the game, because it would be too slow.

The screenshots below show the same scene with flat ambient lighting:

And with an environment probe placed in the scene: