Leslie,
I attempted a reply a couple days ago but I don't see it here. Hopefully I didn't post to the wrong thread.
I wouldn't hesitate to use cedar below grade for your purpose. I can elaborate if you wish.
I'm going to assume that your trellis is more of a pergola with substantial posts and rafters/perlins exposed to the elements.
I wouldn't use crushed rock since the soil is well-drained and the structure should be rigid. I think the pole barn builders use crushed rock to fill around posts because the material is very angular and works better than tamped earth to resist the forces on a post. I don't see how fabric would improve the lifespan of the posts either.
I also don't see how the insul described above would help in your climate (8' frost line: brutal). But I won't surmise further not knowing how the space will be maintained (shoveled or not, hardscaped patio or not, etc.).
Instead of mixing concrete to bring post holes to the same elevation I just use road gravel (easily transported in 5 gal buckets for small jobs). Dig post holes a few inches too deep and bring the bottom to elevation by by tamping gravel in the bottom.
Brook
