I ended up getting a jug of mineral oil from
TSC. Poured about a tbs of sulfur (the yellow, stinky garden stuff for bugs) into the jug and shook it up (didn't really dissolve, but dispersed pretty well). That night, when they were all asleep, I went into the coop and poured the oil into an old plastic ice cream bucket. Lifted each bird up, dunked the feet into the oil for a few seconds, then lifted a bit to let her drip off, then put back on the roost.
The rooster is the only one who gave me trouble, boy did he flap. I did manage to get him dunked, but I think he lifted one leg and hung it on the edge of the bucket so maybe not completely covered that leg. He was so agitated that I didn't put him back on the roost but just put him on the floor. Put lid on the bucket and left. It was maybe 3 am and the pop door opened at about 5:15. No one was sliding around too greasy, but they all had greasy undercarriages. And, funny, the eggs had an oily sheen. That was Sunday morning and I was going to do it again this morning and then didn't (I have a house full of guests, can't do everything).
I plan to dunk the legs again tomorrow morning and then at least once again, maybe every other day? It was much easier than I thought, except for Roscoe, and I expect he'll get used to it. He's a good boy, I just rarely handle my birds.
How often and for how long should I continue? If it's lengthy I may just switch to castor oil or campho phenique.