See
This post at the Gentoo Forums.
Basically, you need to use transcode and ffmpeg to split the audio and video and then mplex to convert back into an mpeg format. dvdauthor will then create the correct DVD structure, and growisofs will burn it to DVD.
It is quite complex to read through it, but it is actually really simple when you understand what each step is doing.
I hope this helps
--Ian
