Well, I've never really had a rat, but I think that the wood will be fine for them. If they can live in the sewers, they can definetly live with wood that has a little bit of a strong smell. Plus, it probably will not be that bad on them, they might not even notice. As for the Asperagous ferns thing, I wouldn't risk trying new herbs on rats, they can be very sensitive to things like that. I don't know of this information is useful, since it has been a day.
Good luck with your rats, I love rats!!!!