Linseed oil works. I went pressure treated for the very same reason you are wanting to treat yours.

Where abouts in NW Washington are you? South Snohomish Co. here.

Here is a link that might be useful: Sinfonian's garden adventure

Sinf, does anyone ever chat on the PNW forum? It always looks quiet to me.
Linseed oil is the only one I know of. We had a barrier in the yard around the playset made out of composite and it rotted from the inside to the outside within 18 months, so I figured, why waste the extra cash, lol.
Some will worry over chemical leaching from the composite, just fyi.

I've posted there before. Too much flower discussion for me though, hehe. As for "chatting" that never happens there. And this is one of the more active forums here.

Ughman, if I were you, I'd head to your local neighborhood independent nursery and start talking organic pest contol to see if they're into organic. Then ask the knowlegable ones what they recommend. Linseed is definitely one, but there. Are likely others.

Though Douglas Fir is a much better starting place than pine. It will last twice as long untreated. I used untreated Doug Fir to build my potato bins. They still look goof after one season while the pine 2x2 is toast. Of course my bin got put away in fall, so I'd treat whatever untreated wood you get. Oh and Doug Fir is cheap too.

you cant go wrong with a product called ECO WOOD TREATMENT SHERWIN WILLIAMS OR HOME DEPOT CARRY IT

Tung oil is what you need. Commonly used in butcher blocks, food safe. Another option would be to use dense woods (ipe aka ironwood, or Teak) to add longevity to your project. Unfortunately those woods are expensive.