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).
I'm using StringTemplate as my decorator language but this should work in Freemarker or any other templating tool. I add in a PageMap 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