Practical Cycles has a good selection of stuff.
I'm not aware of anything that you can get an endomorph through the back of. I've run 2.5" tyres in the Big Dummy, although you need to get hold of the 29er spacers for the rack or else the tyre gets a bit mixed up with the bottom of the board. The Big Dummy is supposedly not too sensitive to changes in wheel size, so you could probably go down to a 24" wheel at the back and put the most humongous tyre you could fine in there.
It depends what you're doing, of course, but for anything on a "normal" spectrum I'm not totally convinced that endomorphs would be much fun to be honest. The Dummy is massively heavy already, more so if you load it up, obviously. Moving is pretty hard work a lot of the time, and I can't help thinking that a very soft 4" tyre would be murderous. It's also very definitely already an easy and stable bike to ride on snow. The long wheelbase, very balanced weight distribution (or some other factor) definitely makes it very steady and confidence-inspiring on snow.
A while ago the "Riding the Spine" blog had some stuff about a heavy duty Xtracycle variant, I don't know if that's going anywhere.
Might be worth a trawl. There was also a thing called a "Hammer Truck" which blew the Dummy and everything else out of the water in terms of its load rating.