Check out the new theming API in Seam CVS (and read the docs and look at the blog example).
I'll think about themes some more. I'm not sure what the implications of replacing the template.xhtml based on a theme is going to be in my application. It might work; but I don't want to add another thing to my application if I can modify pageflow do what I want.
To that end, I created a small patch to make Page.getViewId() interpolate the value in viewId.
If there is any interest in the patch, I put it into JIRA.