I've just come across an (in progress) VLC Google summer of code project at http://wiki.videolan.org/SoC_2007_Project_Dynamic_Video_Overlays to allow a program, via an api, to overlay images over the top of video. (VLC is a powerful video player which accepts sources from DVD's, camcorders, internet etc.).
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No idea technically of how feasible it would be to integrate this into openlp, or whether it will be useful (since it looks to be aimed towards overlaying images rather than text) but thought I would post it in case there was potential for integration in a future version of openlp.org