Steve,
I used to run a tri-level space hulk event at conventions, and here's how I did it. Simply divide your table into 3 or 4 battle areas with points of entry corresponding to where they would be on the other areas. Not only that, but you could have one player play each area's defending force if you wanted to increase the number of players for the event. Although a huge 3d board is great for the WOW factor and would bring the gawkers and generate interest in UWZ as a byproduct, the other way is FAR more playable IMHO.