Magnus, I really like the idea! I believe your original idea (posted @ http://www.codeproject.com/KB/linq/LinqToTree.aspx) is even better for the general case - probably better than the solution in the article - it does feel more generic while still being very easy to use.
I'm sure you may have already implemented something like this, but if you precede
with the following, you can begin the operation with a single item.
public static IEnumerable<T> YieldSame<T>(this T item)
yield return item;
Window w = new Window();
w.YieldSame().SelectRecursive(w => w.OwnedWindows);
Again, excellent idea!