I have found these 6” Polyurethane wheels.
They are the closest thing that I have found to provide the comfort of pneumatic wheels and the low rolling friction of polyurethane (PU) wheels. Almost perfect. The problem is side drift and the fact that they last about 500 km as rear wheel. The above pic is after 250 km.
Frog Legs http://www.froglegsinc.com/ wheels didn’t work out as rear wheels. They can be used only as front wheel.
I think the Sky Way wheel http://www.skywaywheels.com/ can be easily used on an A-Bike. It would require some adaptation for the brake drums. Since it is soft, it should not break apart the front fork assembly.
If anybody is thinking about building a “trick” A-Bike, I think this wheel is a good starting point. It comes in several colours, I bought it here: http://www.wheelchairparts.net/CATALOG_c267354.html