in reply to
Output should have multiple segments
I've done projects where I've dealt with this issue, and came up with solutions similar to what others have proposed.
However, ultimately I realized that this was basically wrong, at least for CSS and JS. For both of these, you're much better off serving a single cacheable file, rather than only including the files you need. This plays much better with typical web clients.
As to the general problem, I haven't run into it too often, and usually when I do it's not too complicated. The most common example is wanting to generate the <title> tag in a top-level autohandler and getting the title from the requested component. This is easily done with Mason's method system.