What's the best way to rinse out a BCD which has an Air2 system? Is it safe to hold the purge button down and run water into it and drain it out the BCD purge?

You can rinse your bcd with an air2 just the same as any other bcd. You don't have to worry about getting water inside a 2nd stage, it's the 1st stage you don't want water in. That is why you never push your purge button on your regulator when rinsing it. Since there is no 1st stage on a bcd, there isn't anything to worry about.

Put the hose or faucet up to the opening on the Air2, push the deflate button, and fill it up! Swish the water around a few times, dump it out and repeat it. Put a couple breaths of air into it and hang it up in a cool dry place, and you will be fine!