Nah, found that while searching for the other. I can probably use that, though. The one I'm thinking of had you putting a short instruction at the top of the file (like @include) if you wanted it to be part of the output file. Maybe I'm not remembering this correctly as I can find nothing on it now.
Server Side Includes? (mod_include for apache, mod_ssi for some others)? Not sure I'd recommend that as a general solution today -- for static sites I think you're better of building the static html, and serve that up. With varnish, you might want to look at edge-side includes.