Good luck with trying lice powder in their dust bath (if you haven't already tried this) as what Tim says makes a lot of sense.The chooks will apply it themselves a lot better.
If you do go to your vet,some vets will flatly refuse to use a product that isn't licensed for that species.I've met that 'one'

,then you get the 'one' who will only try it after exhausting all other possibilites

,I've met that 'one'as well.By which time you've spent more money towards their new BMW or range rover,and then you get the 'one' who does prescribe it,after explaining it might not work........and it doesn't

,or if you're lucky it does.I've met him as well....... :lol:
As your chooks seem to be getting them a lot,are you able to work out if they are being re-infected either by wild birds,or another family pet,maybe you could break the link.