You can do something like this in Cocoon:

  <map:transform type="cinclude">
      <map:parameter name="parallel" value="true"/>
  </map:transform>

Walter will recognize this since he and I were tossing this around in
connection to federated searching (just ideas at this point, but a great
acronym - SETH (Search Everything Till it Hurts!)). I seem to remember that
you had to hack the cinclude transformer to get it to work with a dynamic
source, but in 2.1, the docs suggest this is now built-in, see:

http://cocoon.apache.org/2.1/userdocs/transformers/cinclude-transformer.html

At its best, cocoon would give you parallel processing and caching. This
would be a tough to build on its own. Those apache folks know their stuff.

art