What's a good way to support a freestanding chimney?
My first inclination would be to install a concrete footing 3 or 4 feet deep by maybe 1 1/2 to 2 feet square. Put a concrete block or paver in the bottom of the hole to support the pipe. Plumb the pipe (2 or 3 inch galv should work well) with some temp 2X4 braces, then pour concrete into your hole.  Be sure to crown the top of the footing a little so water will drain off and not pool around the pipe. The reason I like a large footing is to supply good ballast and avoid any frost heaves.(if you get ground freeze in WA.) Also, don't forget to put a cap on the top of the pipe so water will not fill the pipe. You may also want to paint the pipe with Rustoleum before attaching the stainless straps to the flue. This will help avoid any reaction from dislike metals in contact with each other.