I doubt it is the woodlice doing the damage. They do eat a little green material but, as their name suggests, they are more interested in dead and decaying plant material such as rotting wood, dead leaves etc..
Sawdust will work up to a point with deterring slugs and snails but I bet they are the cause of the damage. Any chance of posting a picture of the eaten bits?
Another organic trick you could try to keep slugs and snails off is to cut the bottom off a plastic bottle and slip that over the top of the seedlings. It takes longer for them to get in and if they do it is easier to spot and squish them! There are also pottasium based slug pellets on the market now that are completely organic, good for your plants and they work!!!
No need to reapply after rain, they do the same as the nasty blue ones (attract the slugs/snails) who eat them get tummy ache and have the consideration to go and hide somewhere to die rather than leaving a slimy yuk all over your plants.