I KNOW I've hauled far more manure in our barn cart than I could ever carry. Still, odds are that it was more like 200-300 lbs. The trick is to load heavy in front (opposite the handle) so that the load is balanced over the axle.

Here's another cool wagon that I've had my eyes on for a few years.

http://www.ursawagon.com/index.htm

Has some better trailering options, though it might be more than a bicycle could handle.