An alternative to inner tube - "fix a flat". Supposedly temporary, but that's for cars - high speeds. Fix a flat is messy inside the tire, so once you do it, if you ever take the tire off, it's a real mess.
I think there is a variant called "slime" that's more heavy duty, but doesn't add the weight of foam.fillers.
http://www.slime.comThere is also a foam that is used by farmers on things like tractors, and industrial (forklifts) so they never have to worry about flats. Kind takes up the stress even when air pressure doesn't. Downside (for your application) is the weight.
http://www.noflat.comhttp://www.fixaflat.com/http://www.otrwheel.com/ff.htm 
