So my hen got an injury that I'm 95% sure is from mating, but it's much worse than the one in the original post.
It's like 2 inches where the skin got pulled away from her body. I kept her inside overnight and the wound seems to have healed a lot in that time. Yesterday I cleaned it with a Betadine solution and then applied a bunch of triple antibiotic ointment.
I repeated this (clean then ointment) this morning and this evening.
I'm also wondering how long I should keep her away from the roosters. I'm not too worried about bullying from the flock---the injury is totally hidden by her wing and before I discovered the injury she was doing just fine in the flock and it was a total fluke that I even spotted it.
What I am worrying about is a rooster trying to mate with her and possibly reopening the injury.
At this point she's been in my house for about 24 hours. Normally I'd put her in one of my "quarantine" zones (ie dog crates in the run), but I've had two chickens killed by rats over the last few weeks and I'm too nervous to put her in a vulnerable situation.