I guess if you just combined an air-ride suspension with full rubber tyres, you would get low rolling resistance and comfortable ride in one package. Additional advantage is that you can adjust the air ride suspension to any level of comfort - or the opposite, not to waste kinetic energy in such useless things as shock adsorption - you like.
A completely controllable air ride system for a vehicle like mine costs about 6000 Euros (see
Vollluftfedern - Goldschmitt techmobil AG, page is in german, unfortunately), but this is nothing exotic and readily available.
Full-rubber tyres should pose no problems, too... they could even be regroovable and last for a really long time :-)
so long,
tinduck