Hey, great idea...
I took your idea and modified it. I'm an avid paintballer, so I had some stuff lying around, for all of you who are going to go get ripped off on 12gram co2 tanks you should probably just think about getting full size tanks with a paintball asa.
This is what I'm talking about
It doesn't have to be those exact ones, but as far as I know, all of the on/offs and just direct air have standard fittings that you can stick those brass fittings in with the needle valve. I would recommend grabbing a cheap On/off though because you'll be able to fine tune it better (unlike my set up which is just direct air) but either works well.
Here is an example of what I made - it ended up like 6 or 7 bucks total for me because I already have the Co2 tank and ASA. Make sure to get some teflon tape to ensure a tight seal.
Again, thanks for the idea! I just run this straight into my diy diffuser now for my 10 gallon tank.
Edit: Also, every co2 tank has the potential of liquid co2 in it. Depends on altitude, temperature, and pressure. But, all Co2 tanks should output 850-1000+ PSI whether it's a 12gram or a 20oz. Output pressure will vary based on altitude, temperature, etc too though.