Just an update. We got quadro-sync to work and the engine also works in our cave with head tracking, we are in the middle of upgrading our cave but after that I will try to post a video of it.
Another good news is that our patch to enable support for quad-buffer stereo in OpenGL was integrated in the engine and will be available in 4.8 (see info about it here: https://github.com/EpicGames/UnrealEngine/pull/709) Mind you that it is not a complete solution. It just enables support to render to the left and right buffer from your code, you will still need to write a plugin that will make use of it.
We also spotted a bug with temporal AA and off-axis matrices, it’s not yet in the engine but if anyone is having this issue you can try to integrate it from here: https://github.com/EpicGames/UnrealEngine/pull/912
I also was able to sync the simulation time for all the slaves, this required some engine modifications. And in how the engine is set up the modifications are more of a hack that I don’t feel comfortable to make a pull request for just now. When I have time I will see if I can make a more elegant solution so I can also make a pull request for it.
We also have a little VRPN plugin that maps tracker and button devices to UE4 key mappings, if anyone is interested in it we can see if we can get the code cleaned up and posted on github.