How can I intercept and filter all frames coming out of SteamVR?
So far, I've been able to attach to vrcompositor.exe with RenderDoc and intercept the D3D calls that do the composition. According to Baldur Karlsson (the author of RenderDoc), it's not that hard to hack RenderDoc to use its hooks to run arbitrary code on each Present call of the chain I'm interested in.

I've not had a chance to try that out yet, so this isn't a final answer! I'll update this post with any further progress.