Is there a way to apply templating on the document itself?

[Abastro]

Hello,
I found pandoc's templating engine quite powerful, so I want to use it for more stuffs.
Is there a way to have a document as its own template?
I want to

• Put in placeholder variables which are filled in by metadata, and/or
• Have partials in it so common patterns can be factored out.

Is is possible, or is there an alternative ways for doing these?