A trials bike on a motocross track is like a getting a motocross bike to do trials. You could do it, but why ? I can assure you if Doug Henry was clearing all the jumps, it was not a stock bike. I raced motocross for several years and there is no way I could keep that pace on my trials bike, not to mention the start... I would be last at the first turn. Even trail riding with friends on big bikes is not fun unless you just ride black diamond trails or your friends ride slow.
No, a trials bike will not work. They aren't designed to go fast. I can bottom the suspension of my trials bike just bouncing on the pegs. You will never be able to keep up with your friend on the trails unless the trails are very challenging (then you'll wait for them). I think the bike you want is the scorpa T-ride. I'm not sure when it'll be available. I'm waiting for one too. http://www.scorpa.fr/openswf.php?id=152