Well, in theory, you could use a 3-way normally closed solenoid in reverse. The way we use them with high-pressure CO2 (from cylinders, through regulators) is port one (supply) is opened to port two when the coil is energized, allowing CO2 to flow. When the coil is de-energized, port 1 is closed off and port 2 is open to port 3, which we can either plug or just use a check valve from keeping air or water from backflowing through the tube, into port 2, and out port 3.
The way you could do that with DIY CO2 is install it in reverse, with the supply going into port 3. So when the solenoid coil is energized, port 1 is open to port 2, and CO2 follows the tubing to the tank. When it's de-energized, port two opens to port 3, allowing the CO2 to escape.
Most solenoids aren't designed to work this way, but with the really low pressure involved, it will probably be fine. But there are solenoids that are designed to be run this way. They're multipurpose 3-way solenoids, and installing them backward (port 3 supply to port 2 output) causes a normally open state.
Since you're using DIY CO2, this is probably far more involved than you want to get. But you asked if it was possible, and I answered