Dual posted here and StackOverflow:

If you don't mind extending Sitemesh 3 then this is fairly easy to do by
adding support for server side includes in your decorator template. I do
exactly this in another library (UtterlyIdle<http://code.google.com/p/utterlyidle/>
).

I'm using StringTemplate as my decorator language but this should work in
Freemarker or any other templating tool. I add in a PageMap<http://code.google.com/p/utterlyidle/source/browse/src/com/googlecode...> and
then in my decorator template call

$include("someUrl").body$

This does an include and then parses the output with the Sitemesh 3 engine.
This allows you to have as many includes as you like.

Hope that makes sense

Dan