Or rent a full beer keg (German of course). Drink on it til its dry and then strap it to the back of your bike. The cost of the beer and forefit deposit on the keg will be less than the tank.
Best thing is you wouldn't have to come up with any elaborate plumbing. Just position the tap over the side fuel cap. When it gets thirsty, just open the cap, give the keg a coupl'a pumps and fill'er up.