i'd use steel fence posts. once they are set you can cut them short if you like. you can then bolt the vertical wooden supports to the metal posts. if you use the round chain link fence posts they make brackets for attaching the wood.

I used the recipe in the Vegetable Gardener's Bible, which is 1/2 cup linseed oil and 1 oz melted paraffin in 3 1/2 quarts of turpentine. I only have one season on the wood I treated this way, but so far so good!

I've read of a tungston oil, don't think I got the name right because when I searched it, it came up as the metal tungston. It was used regularly on untreated spinning wheels, will keep searching for the right name, but I'm sure that is it.

There is also something called Teak Oil. I posted the url below.

Also, don't think it's very organic, but I read about people building pole buildings and preserving their wood with kerosene. Surely kerosene can't be good for organic, but it's a thought.

I hope that helps.

Here is a link that might be useful: Teak oil

I think maybe you mean tung oil. I guess it can be used like linseed without changing colors (or so says the almighty Wikipedia).

For wooden trellis bottoms, I've actually seen people attach removable wooden "legs", so that as they rot, they can be replaced easily, keeping the real trellis legs from ever touching the ground.

Here's the link, if you wanna check it out:
plans for trellis with removable legs

Here is a link that might be useful: Lifehacker trellis

I do not think tar would be toxic after it dries. I put it on the very bottom of my trellis's and use a smaller piece of wood to stake in the ground and screw my trellis's in.

Exterior latex paint?

I found this product two years ago and used it on my untreated wood boards for my veggie garden. You take the packet of dried herbs soak them in water then paint the greenish mixture on your wood and when it drys you have treated wood! :) Worked two years for my raised bed so far.
http://www.ecowoodtreatment.com/

If it's an option for you, line the wooden planters with something that doesn't rot or allow water through, like black plastic nursery containers or pond liner. That way, the wood isn't in constant contact with wet soil. I know that may not be a good option for everyone based upon the design of the particular containers in question.

Thanks for all of the ideas. I'm not sure yet what I'm going to use, but now I have more options I hadn't thought of. All of the info I found about the oil and charcoal method seemed to come from the same source. It was repeated a lot, but no info about actual experiences with it. So I may try one of the other methods instead.