Ah yes, linking fields to expressions (or to load script)...
As they say in Sweden, lycka till. (Good luck). This is an area where we (Qlik) are working hard to facilitate. But for now, it is difficult at best.
In the Governance Dashboard, we made the decision to limit the effort spent on linking expressions to fields due to the resource requirements (load time) and the fact that expressions can be full of noise (white space, comments, etc.). The closest we provide in the Governance Dashboard is basic text parsing of expressions, dimensions, and variables for fields. You can find that on the "Field Usage" sub tab on the Objects sheet.
The Field Usage should give you some guidance, but it should not be considered complete. In other words, do not start removing fields from apps because you can't find them in the "Field Usage" sheet.
As far as the Governance Dashboard, it pulls metadata from the QVDs (xml in the file) and the QVWs (xml footer).