As I understand the dangers of pressure-treated wood, tests seem to show that the leaching does not generally spread more than a couple inches away from the wood itself.
If you do use ivy, please do not use English Ivy (Hedera helix). It is a nasty invasive plant which can spread via seed (birds) if allowed to grow tall. See http://www.oregonlive.com/portland/index.ssf/2008/10/nonnative_english_ivy_is_poiso.html or just google "English Ivy".
Kathleen Sanderson wrote:
If you are in coastal Oregon, I've seen ivy growing up trees and power/phone poles (where it wasn't removed) to great heights. If you could fasten a wire shape up there someplace, you could make an oversized ivy topiary!
I think I will do something like post a few birdhouses/bathouses/butterfly houses up it