Steven, we have Wikimedia Canada, which is larger than Cascadia and includes multiple provinces.
Yes, chapters can be on the national level. Most are. The theory from the Committee goes that you can have a country or a lower level form of municipality like state, province, or city. But you can't span multiplie countries, and you can't spread out across a selection of subnational munincipalities.
To be clear: I'm not opposed to the idea of a Cascadia group of Wikimedians. I'm very much favor of it. I'm just warning you about what I've heard about the bureaucratic limitations imposed on official chapters.