Here's a feature I just hacked together for XoxoOutliner and plan to refine further:

Not entirely sure that this is how I want this to work, but these three URLs demonstrate the ability to address and fetch subsets of outlines. I'm hoping this will be a basis for selective transclusion in other outlines, or maybe even in a sidebar of a blog. (Which, depending on the blog software, might be built from outlines anyway.)