I don't see it would be a problem to keep helium from escaping from your tubes. Surely the relevant "bladder" inserted into the tube could solve the problem. However, do agree that it's a waste of time and money. If you look at the helium balloons they sell in shops you'll see that even a fairly big one can be weighed down with something as small as a 2p piece, so I reckon a couple of tubes full probably wouldn't even give you 2-3kg. I think this discussion is just a lot of hot air............ah, hot air, there's an idea?
You get what you settle for!