One possibility would be Acer circunatum (i.e. Vine Maple) "Monroe" cultivar. It's native to the Pacific Northwest, and has the dissected features of a Japanese Maple. Though this is a cultivar (and I can't speak as to the cultivar's hardiness compared to the species), the typical Vine Maple is far hardier and tougher than a Japanese Maple. Not sure if "Monroe" is multi-trunked--the species certainly is.

For a good comprehensive website of Japanese Maples, check out Mountain Maples (Calif.).

Another tree which might work is Styrax--the Snowball Tree, which is pretty hardier and beatiful. Not multi-trunked, though.

I live in the inland northwest and I've had good luck with Acer circinatum. Here is a photo of a multi-trunked vine maple I recently planted. The leaf shape of vine maples is quite variable, from deeply dissected (Monroe) to almost round. It's hard to tell from the photo, but mine has deeply divided leaves to the point that most people think it is a Japanese maple.

Here is a link that might be useful: {{gwi:1007928}}