The wood ash won't do for long term...the lice come back. You might try sulfur dust or pyrethrin dust, both are natural and safe to use on food animals.
Sulfur dust can help, as can lime, working it fully into the feathers into the skin, making sure you coat any of the egg clusters around the vent area. You could even coat those with NuStock or castor oil to smother and kill them. Dusting the roosts with sulfur or lime may also help, as well...
Lice are pretty much water proof and so are the eggs, so you may rinse some of them off the bird in a bath, but I'm doubting it is an effective, final solution for the parasite.
I wouldn't wash a chicken..it removes all their natural protective oils on skin and feathering. Best to use what they use for cleansing and parasite control and use a dust of some kind. Sevin is not organic, nor is it anything I'd put on my chickens but there are other things out there with...
Why not just mix some wood ashes in your deep litter and arrange a nice dusting box with the same right there in the coop? The chickens love them and the lice and mites do not. I'd even venture to place some wood ashes in the nest boxes...couldn't hurt.
As for the roosts....there is a thing...