I've done something similar in the past by piping xvfb output to ffmpeg. Saves you from having do deal with window geometry.
Was that not an option?
Honestly, I haven't tried Xvfb in years. That said, I did have another motive by keeping things simple. Even though xwininfo is very X11 specific, I hope that it's easier for people to work out an alternative for other platforms and adapt the recipes from my post to their situation.