Meggamouse - and formerly Velocirippa has kart tyres that are technically foam filled but the filler is like a jelly.

Advantages;
1 - behaves like an air filled tyre at about 28psi.
2 - impossible to puncture - they have countless holes in them and a previous tyre had a wedge sliced out of it and it made no difference.
3 - when you change tyres the filling comes out in one piece and you put it in the new one.

Disadvantages;
1 - extremely heavy, MGM tyres weigh 2kg each.
2 - needs to be filled professionally, and if you have aluminium hubs, the pressure (usually air) is still there if the hub cracks, and will eventually destroy itself. I now have nylon hubs.
3 - and probably the worst, a friend got them done for nothing through his company. He was made redundant shortly afterwards, does not know the company that actually did the work, so if I needed any more it will take some detective work.

Back to advantages; When MGmouse was built the only thing I kept from Velocirippa was the motors, gears and the wheels.

On a feather the weight may not be an issue but if you want me to do some searching I will.

Trev