Having multiple transform steps/layers is a nice idea, indeed. But <xsl:include> isn't the way to do that - that's XSL core and cannot be used to create different transform rounds. Diazo is applied via plone.transformchain and for that to work, another diazo transform has to be registered.
What I'm asking myself is, do we really need this method="raw" in https://github.com/plone/plonetheme.barceloneta/blob/master/plonetheme/barceloneta/theme/rules.xml#L43 ? This makes it effectively impossible to modify the content before it is copied via the <replace> rule.
/cc @ramon @Albert @sneridagh