120/208V to single phase 120V
There is no issue running an unbalanced load through the breaker.

I would probably make the adapter, since code inspectors have no business looking at what's plugged in to an outlet.  They should be concerned only with what is part of the fixed structure.  You can always unplug it when they come around.

But that kind of outlet makes me think it's a business with a compliance department and lawyers.  Even if the inspector doesn't have a right to complain about your bootleg adapter, your insurance company will if there's ever a fire, related or not.  And personally, I don't like shared neutral-ground conductors.

What's going to really get you though is putting a 20A outlet on a 30A breaker.  If your AC failed in a way that caused it to draw 30 amps, it could overheat that outlet and start a fire in the wall.

So.  You're going to want to change the breaker, and when you do, I would recommend you rewire as follows:

1) Use the existing combined neutral-ground conductor for ground

2) Pick one of the phase conductors, and clearly mark both ends with white tape.  Use that for neutral

3) Pick another of the phase conductors and cap it with wire-nuts on both ends
4) Take the last phase conductor and put it on a new, 20A single pole breaker.  Install a 20A outlet for the AC unit.

For extra credit, take advantage of the wiring in the wall to bring TWO 20A circuits to the box where the AC will plug in.  Steps 1&amp;2 as above, then.

3) Buy a double-pole breaker.  Be advised.  In a 3 phase panel you need to particularly select a 120/208 double-pole breaker, as opposed to just a 208 double-pole breaker.

4) Wire the two remaining phase wires to the double pole breaker.

5) At the outlet box, install two seperate 20A outlets.  One on each phase, sharing neutral and ground between them.  If it's a single gang box, break the tab between the two hots.

This will let you dedicate one 20A circuit just for the AC and still have plenty of power on hand for other uses.