Encodes system audio on fly to mp3||aac||ogg with slow gating, interleaved with variable frame rate video where frames are generated based on changed pixels on the screen and the delta is stored in such a way that when playing and seeking in the file, the seeks are made to the audio stamps and then the image is rendered by backtracking to "keyframe" and forwarding through the deltas to the audio stamp.
So if you capture say infinite hours of audio and video where nothing happens and ~5 seconds of audio buffer statistically appears to be just noise, the resulting playable video file size would not change until something "statistically significant" occured on the audio or video.
Something like saving the stream from a "remote desktop" protocol to disk might do the trick as long as it could be implemented locally without enabling remote desktop.
Requirements for blocking:
Software which does not have administrative privs should only be able to capture its own window content and if it's a browser then the any activex plugin should be restricted by the browser to be able to only capture the tab the plugin loaded from while the user visible URL stays the same. I don't really care if existing apps break due to this change, those apps that break could be whitelisted in HKLM.