Anyway, don't know whether this stuff is still available, or whether it is
possible to get it in the kind of width tat would be needed for a
wheelbarrow tyre, but it may be worth looking for.

A relative used to have a bike shop and sold stuff called 'slime'. 'Orrible
innit?
Filled the inner tube with goo so it couldn't puncture.
Also sold solid bike tyres (as seen on Tomorrows World), the inner core was
like a soft honeycomb. He said they were lousy on a bike as they were so
heavy.
DaveK.