I know he used to sing backup quite a few times. I have a few shows on video from '82, '83 where he sings during "Two Hearts Beat As One" ("I can't stop the dance!!") and also during "Sunday Bloody Sunday".
They have him all dressed up and clean, but if you notice, he's wearing these huge CAT work boots, I don't have any stills of them, but you can clearly see them when him and Larry are sitting on top of the angel.