Having worked with MR, and a bit of Vray, I would think it near impossible to shoehorn them into rendering TG scenes. Displacements are not the strong point of Raytracers... So much data must be in memory to calculate GI, that I would think they would choke. That said, you are doing some form of raytracing/GI in TG, so perhaps not impossible. But Scanline renderers would have much more success at rendering displacement-heavy scenes like those in TG.
I'm curious to know how TG is doing it's raytrace functions without blowing out memory.
In general, integrating a 3rd party renderer is no small task. Especially if the application was not designed to plug in renderers. To this day, Maya/MR integration needs work;)
Plus, it seems the renderer in TG is the breadAndButter of the app. It's not something that needs replacing... just tweaks, and proper documentation. I'm sure once the tools are made clear, we'll be able to use them more effectively.
Andrew