I'm thinking how you would do this. I'm thinking you would have to take the A/C load completely out of the box and put it in your mini box. Now put a 20 amp breaker in the mini box and run #12 into the breaker that was used by the A/C originally. now put in another breaker and run the wire to your "other" 20 source. I'm thinking you would want to bring that in to a breaker NOT to the bus. Then put in a 3rd breaker and hook up the A/C load.
When you want to use the A/C off the "normal" 30 amp feed, the breaker to "other" 20am source would be open while the other two would be closed. When using the A/C on the "other" 20a source, you would open the first breaker to the original box, and close the other 2. Personally, I think I would "open" the breaker in the original box just before I did anything with the other source, just so you don't forget which breaker is which.
If you have a dedicated breaker for the Mic, I might also put that with the A/C so you can use all you counter stuff and the mic too.
Am I confused yet???