Are their any N or CO2 tanks that don't have to be stored sitting up?
It is possible a CO2 tank can be stored horizontally...not sure if this would affect the safety (overpressure) valve or not. They definitely have to be upright or at least at an angle to use for inflation, because the outlet cannot be submerged in liquid CO2.
Here is about the most horizontal the tank can be for inflation:
http://www.powertank.com/products/sf.../productID/304