It has been a long week, so I'm being slow probably...
The Humax PVR has HDMI in, and can record from that input?
I've nto come across anyone else using this approach, so can't comment, but the idea seems sound logically. The PVR won't care what is broadcasting the video, and the DroidBOX won't care about what is "displaying" (presumably a passthrough to a TV after the PVR?) the video data being sent. Only hiccup is HDCP issues, but unless you're watching actual DVD discs or watching video streams from OFFICIAL apps, I don't think that will rear its head.
If you're not planning to record the DroidBOX's video/audio output on the PVR, let me know where I've gone wrong!