For lice you are really wanting to get the Crovect or similar onto the skin, so it's best if you can part the fleece and squirt it onto bare skin.
You also need to be sure you know that it's biting lice not sucking lice - Crovect treats the former but not the latter. We've used Dectomax (injection) for sucking lice in the past.
In sheep not used to much handling, the amount of handling you'd need, so close to lambing, might be an issue and cause more problems than you solve.
I would ask your vet what s/he suggests - they'll know what it is most likely to be and advise accordingly.