I see no problem with splitting off a gas line to be used for additional equipment. I will have to assume that you know what your doing when you say you need a natural gas line for the c02 generator. Not all gas is created equal you know and different parts of the country use different gases in their home. Since I don't know your setup I couldn't say whether it would be easier to split at the meter, behind the dryer or somewhere in between. Some homes, plenty for that mater use no gas and some use everything gas. They make gas a/c, gas stoves, gas everything.

A gas line is serious business and technically you need to be a licensed plumber to even touch a gas line. Most cities require a permit and license before touching a gas line since one wrong move will cause the entire block to burn down not to mention your death either from an explosion or a small leak that goes unnoticed. Enough of that.

Gas lines are easily joined with a compression fitting. Most gas lines are hard copper lines because they want to ensure no leaks. Any leak is your enemy. If you know what your doing and have the skill set then I would recommend doing it like this:

First shut the gas off to the home
Turn on something gas fed in the home and ignite to ensure all gas is bled from line to home.
Cut the copper line where you want to insert a "T" fitting
Use compression "T" fitting(type compression nut in google to get an idea of what I'm talking about) One end will go to the old line for the dryer, one end to wherever the line came from and the final line will be for your new tool.
Once the "T" fitting is installed then connect your new line to the Co2 unit.
Use thread tape everywhere to prevent leaks and make tight connections
Before closing everything back up, turn the gas on and test for leaks. Contrary to popular opinion the best way to test for leaks is not with a match, warm soapy water bushed across all the fittings will show you where you made sloppy connections.

I would advise doing this as early in the day as possible so you have time to undo what you did if you screw it up. You don't want to sleep in the cold tonight waiting for a plumber to show up tomorrow. Home Depot can show you what parts you need if you can find someone willing to overlook the fact that your working on a gas line. Sometimes those clerks can get real upitty when they figure out your doing something you probably aught not be doing. I had a guy totally chew my ass out one time when I told him I wanted to run a water line into my attic. LOL He was crying about the fire department not having enough line pressure to put out a fire and on and on.

You will be fine. Lot's of people replace their hot water heater without too many issues but then again several times a year people die during the same process so be careful.