I was just refactoring some of the GSoC pages on the wiki and came across this project idea:
Currently we use the native platform’s video libraries and codecs, these are frequently extremely lacking in support - whether it is lack of support for webm / ogv on Windows, or various other codecs elsewhere. This leads to a poor user experience, and user complaints: “why can’t LibreOffice play my video”. As such - it would be great to detect if libVLC (now LGPL, with all it’s code goodness) is around on the system, and if so - integrate and play using it. We should do that by dynamically loading and hooking version / symbol information out of it, and provide a new cross-platform, pluggable avmedia/ implementation for the latest APIs.
That sounds pretty similar to what you’re trying to do in Impress. Unfortunately, as it is marked as a potential project for GSoC, that means that it isn’t yet implemented. On the brighter side, LibreOffice is currently accepting applications for GSoC, so it’s possible that someone might sign up to work on this improvement!