I can't tell from the photo how high the ring is off the ground, but my chainstays are only 30cm off the ground, and if the U-lock's shackle isn't grossly oversized, the remaining parts of the ring that didn't break off (the thicker parts where the ring appears to be bolted to the post) would probably prevent a lift-off approach. If the shackle's longer than necessary, snare the driveside crankarm in the shackle as shown in my photo to help fill up the space.

Alternately, with a fairly long chain like a Kryptonite New York Chain, you could put a couple wraps of chain around the post down there below the ring and make sure all available slack is removed, to get a "python grip" on the post. But if I had a New York Chain along, I'd go right for a full-blown streetlight pole. Let's see them cut through THAT!