Adding some regularly to their dustbowl is a great way to get them to treat themselves as a preventative measure. However, in addition to doing that, I would be inclined to treat each bird individually as well, since you know that you have definitely got a problem with lice at the moment.
It's not that difficult to do - get an old stocking, and dump a load of the dusting powder into the toe part, then tie a knot in it. Take each bird off the roost in the night, and 'powder puff' them all over with it - under the wings, rubbing the feathers up the wrong way on the breast, belly, back and back of the neck, and getting inbetween the feathers around the vent. Finish with a light sprinkle over the top of the head, to get any lice running about on the comb or face. I keep the 'powder puff' in a bowl on a shelf in the coop, and any time I spot something on one of my girls I grab it and give them all a quick dusting. I won't say that they enjoy it, but they don't make too much of a fuss, especially if I do it once they're 'in bed' for the night, and are feeling a bit drowsy.
This may be one of the best pieces of chicken advice and hacks I've EVER seen.
A Sevin dust "powder puff."
KayTee, YOU ARE MY SPIRIT ANIMAL.
MrsB