Browsing through some older posts here ...

For this post I experimented with one slide containing 2 videos, both starting automatically:
- first the one I wanted to play as a background video
- on top of that: a wmv file saved from a presentation containing the other slide elements, with transparent master background and automatic transitions between the slides

Unfortunately the transparent background doesn't work out: still shows white background.

Why am I posting this anyway? Just brainstorming - might bring someone to another idea :-)