Well, QLab QC support is still weak.. In fact you can write a custom patch to modify the way a single video is composed. I used to put in this composer a syphon server, but with some issues – even stability issues, when the movie is stopped and played again in short time intervals.
What I am working on is a tool that emulate a fake monitor out, so that you can choose that monitor as the main output in QLab (and other applications), a “syphon virtual screen”. Then this output is piped to the GPU on a Syphon server for further video processing. Sort of what Soundflower does for audio, but for video..