The pressure inside a 20# cylinder is relatively low; much less than 200 psi. However, the pressure varies over a very wide range due to temperature. There must be some expansion room inside the cylinder. A cylinder should never be more than 80% full. If the report from one of the posters above (sorry, I forget who and it's too much trouble to look) is true and you can't get one of the little cylinders more than half full, there should never be any problem.
Myself, I just wouldn't trust it. If the cylinder is liquid filled, and if the temperature increases, the pressure will exceed the pressure relief valve, some gas will escape, and if it's near a source of combustion, it will create a fire bomb. I don't think I'd like even a little one. Maybe I'm too conservative, but I handled hundreds of thousands of gallons of the stuff and never had the whiff of an incident.