Brian may be a genius, but even he can't sing using three or four voices simultaneously.
![Grin](http://smileysmile.net/board/Smileys/default/grin.gif)
The Imagination tracks played in the concert featured his vocal tracks from the album, if I recall correctly. He sang along, but I'm not sure how much of that made it on the released DVD.
One of the reasons given for that at the time was that they planned to use the concert footage for videos of the songs. They ended up doing so for "Your Imagination," but no other videos were released.