I've seen pictures of guys doing this on their webbers for full volume brews, and I thought about doing it on mine before I rigged my electric boiler.
I certainly did a few small boils in a 20L pot on the side wok burner.
Problems you may encounter:
If you've got a hood on the BBQ then it might restrict the space for the pot.
Burners lower down so you'd get some heat loss.
Burners rectangular compared to a circular pot, more heat loss.
How big's your pot? What's it made from? Are you going to use it for anything but brewing?
If the answers to the above are - 30+L, Aluminium & No
Then head to kmart buy 2 x $9 kettles and pull out the elements, drill some holes in your pot, install elements into pot. Got yourself an electric brew rig, no need to worry about gas ever again. Also easy to hook up to PID's etc for doing stepped mashes if you're doing BIAB... Conversion to Braumeister style rig etc etc...
If you pot is Stainless you can still do the above, it's a little harder, and would also depend on the thickness of the walls of the pot. The aluminium pots have a the perfect thickness to use the seals which come with the $9 kettles.
