While it would be convenient to have this ability, the place for it isn't in the template file for several reasons.
1. Changes to the template language would be incompatible with the various other htmltmpl parsers, not all of which are maintained here.
2. That sort of definition really belongs in the template manager definition.
3. Allowing includes from an arbitrary path also allows...
HTMLtmpl - templating engine - can compile templates to native PHP code now. If compiled template is newer than original one then templating engine only includes it and doesn't process the original template at all.
http://htmltmpl.sourceforge.net/.
