Make a document ESEF-ready using InDesign

To be honest, I don't know that InDesign is the right tool for this task. I mean, I can see the rough outline of the development task necessary to pull this off with InDesign, but it feels to be more at the scale of "let's start a company around the procedure of turning annual financial reports prepared in InDesign into compliant ESEF documents!" and not  something that can be covered within the scope of a few posts on a forum.

ESEF is basically XHTML with extra tagging for financial data, right?  That tagging looks non-trivial to me. A bit of clicking around showed me a couple of commercial products that seem to be aimed directly at you, or perhaps at your employer/client. No experience with any of the products I found, so it's not like I'm recommending them. I also found the basic manual describing the format, as well.