I think the main differences between Wikis and Outliners is deeper than the technology problems. It’s to do with mindset and culture.
Wikis are great for expressing unstructured thoughts, and for discovering hidden connections. The first time you make a WikiLink in something you’re typing and unexpectedly discovering that that page already exists is an enlightening moment into the Way Of The Wiki.
Outlines on the other hand require you to structure everything up front. An outline is great for reading, but not so easy to write.
Perhaps software developers find it easier to think in structured ways? Good developers break things down into lots of little subproblems, each of which in turn is a series of other subproblems, etc. Some people can do this naturally. Others write in wider scope, and refactor into smaller chunks later.
Most people don’t write in a structured manner at all.
A true knowledge sharing system needs to cater for both approaches. There should be the ability to jot down random thoughts (a personal weblog). To narrate your work (a personal outline). To record the structure of a project (a collaborated outline). To have a project calendar (a collaborated weblog). And to connect all these in interesting ways (a wiki).
Having a wiki output OPML won’t work, as it requires creating structure that isn’t there. Having a wiki input OPML, on the other hand, might produce much more useful results.