No I would definitely not put them outside where they might/ might not pick up something. Just those little unknowns are what I like to control. No need to let the possums come by and do their thing on it?
Since they have been inside and dry for that long, the curing that will remove any tannins, etc. has been done if it is going to happen. I don't use manzanita and can't really say much about it but this is the way I prep any wood I find. Since all wood has the risk of having been exposed to some things like pesticide, oils or some other unnamed hazard, I treat it all. Maybe a local methhead dumped his trash on it!
Wash the plate before eating just because it's a good idea?
I put the wood into a tub of water and weight it down with rocks, then add a bit of the cheapest, uncolored, unscented bleach. How much is not critical but depends on how large and how dirty the wood. More dirt gives more organic for the chlorine to react with so I adjust depending on what it is. If the wood looks kind of clean, a tablespoon is enough in fifty gallon. But then more doesn't hurt so I make sure there is plenty. At 5-10 cents a cup I can splurge. More and stronger may turn the wood white for a while but the color comes back after it's in the tank.
I let it soak for 8-10 hours or until I get back to it. Longer doesn't hurt but at some point the chlorine gasses off and mosquitos may begin to breed in the tub!
I take it out, rinse it and let it dry until I can't smell the chlorine as it gasses off and then it is good to go.
If somebody tells you the chlorine will stay in the wood, ask them how it can since chlorine reacts with organics.