Oh.. nevermind. Turns out that was pretty easy <img src="smileys/smiley17.gif" border="0" align="middle" />
For others interested:
Give the video instance an ID.
make a document with this text:
#videoID{
z-index: -1;
}
save as style.css. Import to Files in c2. Then make an On start of layout event that imports the css-file through the plugin and then it works.
On another note; is anyone else experiencing that video is shaking slightly unless it's placed at 0,0 on stage?