Jim-
As to puncture-proofing ATV tires, cycle or cart tires, check out www.tireballs.com. Individual cell bladders fill the tire in lieu of a single tube (or tire/wheel seal). These are the hot stuff with the off-road racing crowd, and they pound the **** out of their rigs more in a single race than any sane individual will in two lifetimes!
One other thought on tires for push-pull carts…knobby all-terrain tread patterns may look cool, but unless those tires are being powered, you’ll be happier with a much shallower tread pattern. In fact, only enough tread to keep the wheel turning instead of sliding is the goal. Why? Traction = friction, and friction = increased work! It’s like dragging a cart up the hill with the brakes on. Just a thought. Keep the faith, – Bonehead