Here are the answers I can provide promptly:
Tablet AR vs Stereo Alpha The way the tablet AR sample works does not extend to stereo RGB+A. Because of the OpenVR limitation, server applications can only submit two RGB textures. The current protocol in the sample clients is that for mono RGB+A, RGB is sent through one eye, and A through the other.
OpenVR We know, and we know. Our next generation work is moving to OpenXR, but it simply is not ready for prime time.
Chroma-key on client The clients we provide are sample clients, not meant to be used in their provided form for publication. Chroma keying on the client requires shader level modification to the client, plus as you point out modifying the client to use a pass-through mode, but with the right skill sets, should be fairly straightforward. I have done so with the Unity plugin fairly trivially; the C++ clients will take more work.
So, yes, CloudXR 4 supports RGB+A stereo streaming, but there is not a server that will send stereo RGB+A, neither one published by NVIDIA nor anyone else so far as I am aware.