Does anyone know of any work being done (academic or commercial, I can do the research if I know where to start) to real-time approximate effects besides rigid and soft bodies? I'm looking at building a playground which includes a more robust set of interactions - specifically, thermodynamic, optical, electromagnetic, and chemical. It would likely be helpful to look at the literature on the topic, and indeed I plan on doing so, but I figure some smart guys may know more precisely where I should start :)

There are engineering level codes for all of those disciplines, commercial and open source. In the game dev world, though, I haven't seen any of those. Some examples:

Optical Research Associates CODE V and LightTools

ANSYS MultiPhysics

Algor Multiphysics

I found this interesting website, though I didn't explore it enough to know what its all about:

Open Source Physics

And this website provides tons of links on the finite element method, which can be used to run simulations in many different disciplines.

Internet Finite Element Resources

I will make one final comment....you wrote "robust set of interactions." That is difficult with a single discipline such as rigid body dynamics, and far moreso when you have multiple disciplines involved. I don't think you are going to find any existing solution that provides something as robust as you would like with all the disciplines you want. But, I think a playground such as the one you are attempting is possible, as long as you isolate the disciplines, just coupling them in the boundary conditions, keeping those super simple, and making sure that you use a simple enough representation within each discipline that each is unconditionally stable (which in itself might require severely limiting your feature set).