Thanks - that may be a way to deal with my hybrid requirements; I have to make the solution work with a 30 year legacy of existing pages that make use of apache functionality in various ways.

Whatever I do has to apply incrementally as I (slowly) transfer everything into the new SSG controlled repository.

Normalised is definitely where I want to be for the canonical representation.

I guess there'd be two canonical representations actually; the SSG 'source' and the normalised hybrid HTML+SSI.