From: Atlanta, Georgia, US
ChrisinSunnyside is correct about argon - easily obtainable from Airgas, Amerigas, Praxair, your local mom-and-pop welding supply, etc. Personally, I would not bother with Coravin, as the only critical thing here is the needle that will allow the wine to flow through it, while being thin enough to allow the cork to seal the hole it leaves. That too is not a problem, as long as Coravin's claim that the cork will indeed seal itself is true.
To hack this deal is absolute child's play. The most efficient way to do it, or just to get the same thing that Coravin promises, would include what Dave said:
you would need a second needle
In the drawing, I'm including a needle for liquid flow that's long enough to reach all the way through the cork and down to the bottom of the bottle. It could be placed just above the bottom, or just above the sediment. Obviously, it would not have to be that long, and a much shorter needle could be used if one would tilt the bottle.
For work, we use needles that I'm sure are no thicker than the one Coravin uses. Other ones are much thinner yet, and that would be "B," which only needs to allow the passage of gas, for pressurization inside the wine bottle, or for evacuation if one would be refilling the bottle with liquid. We've got needles where the outside diameter is less than that of a human hair. Cork sealing would not be an issue, here. Connecting the lines to the needles is also not a problem. Luer connections are available, or compression fittings could be used, as with the 'Swagelock' style, or other methods yet. We routinely use lines that are 1/16 inch outside diameter, with the inside diameter being .03 inch, and that would suffice for the gas supply line.
In practice, the argon cylinder would have a valve on it, and then a pressure regulator installed, the line to needle B connecting to the regulator. An additional valve between the regulator and the bottle is probably advisable. Argon is put into the bottle through needle B, and liquid flows out of the bottle through needle A and into the wine glass. Argon from a supplier, as above, will be virtually free compared to Coravin's prices.
To add liquid to the bottle, it's pumped in through needle A, with needle B venting outside the bottle, rather than being connected to a gas supply.
I have no doubt that stuff like this is going on right now, and I'm wondering about the capsules on wine bottles. Needle A or Coravin's needle would make a noticeable hole in the top of the capsule if it's intact and on the bottle. Needle B can be so thin that an almost microscopic "tear" could be made, to get away from the round hole the needle would make, and to disguise it. From now on, if I see a capsule with a hole in it, or a cork with anything that looks like a needle might have been run through it, I'd be wondering...