A new release from developer Zone-MR now allows the use of AirPlay Video in all third party apps, including Safari. The app is called AirVideoEnabler and is a free from Cydia. This means content from games, art apps, the native YouTube app, etc. can all be streamed directly to your iDevice or Apple TV or other enabled sound system or docks.
Update: you need to add the repository http://cydia.zone-mr.net/ to download the app.