I would say no way. You won't find a shop to fill it. Legalities you know.
And as far as I know there is a dip tube inside the Co2 tank that may prevent it from mounting horizontal.

Fill with CO2 - no - The fittings are a universal size for all CO2's and I doubt it would matchup. Not to mention no one would do it for you.

CO2 sideways - no - its liquid. You have to leave the gaseous area in the tank at the valve. Even when upright things can freeze up fast on a full tank.

Here is a CO2 bottle that is made for what you want it for. You can run air tools off this baby. High priced but here's a link:
PS I apoligize, it's a Jeep catering site.
Quadratec-powertanks
First, I realize that Bretwalda already posted the specs for these, but why would you want to half ass something that could kill you very easily? Powertanks are made for off roading use. They have the proper regulators ect. No questions asked when refilling, all safty documentation...
Second, I would not mount a pressurized tank outside the vehicle, an accident or a rollover could get complicated quickly <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/zombie.gif" alt="" />
It's kinda like saying you want to bolt an aux fuel tank to your rack. <img src="/forums/images/graemlins/scared.gif" alt="" />

Indeed. If you drive that down the highway, you might end up getting pulled over by the state police. I'm not sure, but I think there might be a requirement for a permit and placards when carrying high-pressure tanks of any sort externally on a vehicle (depending on state, of course). I'm sure Kalifornia requires permits and placards for just bringing a bottle of Wite-Out« home from the store. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />

Better solution vs. Powertank: thread link

There is nothing special about Powertanks tanks - except for the paint. <img src="/forums/images/graemlins/rolleyes.gif" alt="" /> The regulator - while it will allow more CFM over a conventional regulator - is waaaaaaaaay overpriced. I use mine everyday (which is designed for higher CFM) - works good. Has a little problem freezing with a sandblaster on a full tank (I think the CFM requirement on the blaster is close to 7 CFM) but I would bet a hundred bucks the Powertank regulator would too.