Shields can have resistance added. For instance, I seem to recall a shield with 30% puncture resist. I think a shield with a resist gives the resist everywhere, much like a suit of armor gives resist everywhere.

Could 620 be made to work with wooden shields, sort of like it works with leather armor accessories? There are a bunch of cool magical woods, like villswood, really a bunch of them. It would be cool to have a mechanical niche for them.