Under the tail, but otherwise not enclosed? Probably okay -- but make sure it's very securely mounted -- not just worm-gear straps or plastic tie-wraps, but rather clamps bolted to the frame of the tail -- or something like that. Put a shield on the valve to prevent damage and instant depressurisation in case something nearby goes bad. Use a SS hose -- not just light rubber line.
If you bring it to me for inspection I'll probably still check with other inspectors to make sure we all agree that it's safe, but I think you'll be okay. By the way -- please consider labeling it "CO2 for air shifter" so the emergency workers, should they be needed to assist you, don't think it's a nitrous bottle and be worried about it.
