1) Have a city near the HC that is well defended (you don't want to accidently lose it early).
2) Set tax to 80% so your LOY settles at EXACTLY 20.
3) Spam attack garbage hits to the HC in order to try and bait a retaliation attack.
4) See the birdie flash and double check the hit is coming from the HC.
5) Destroy all wall defenses, and send all troops away so the city is defenseless.
6) 10 - 20 seconds before the attack from the HC lands, levy anything to drop LOY to exactly zero.
7) The HC player caps your city.

The player cannot abandon or lose his new city, because of the recent "last city" fix. You may now capture the HC. If he is smart, then he won't take the bait. He will NEVER attack another player. But, you can hope that maybe he is foolish enough.