Quick post to correct an error I made!
I stated previously that I was using Frontline Plus on Agatha to treat her lice, and I said the main ingredient is Ivermectin. THIS IS WRONG. I was confusing it with another product. The main ingredient in Frontline is Fipronil.
Both are safe for use on chickens according to The Chicken Health Handbook by Gail Damerow.
BUT Frontline should only be applied topically, on the back of the chicken's neck or between the shoulders. Ivermectin can be used topically or orally depending on the solution.
I'm doubly confused now because I Fipronil is used to treat fleas, face fleas, and ticks, not lice, but Agatha has had no sign of lice since her treatment! I will continue to monitor her and report back.
HUGE thanks to the listener who pointed this out to me, and my sincere apologies for the mistake!