yeah, there is nothing in the PG database that identifies the internals of the XML metadata (unfortunately).  I've asked support about this in the past, and they said to download each workbook and have a look, yeah not so scalable.

In any case, see the attached thread.  You may be able to search for "relation name='Custom SQL Query'".  It depends how often you want to do this as the below solution is really more so a non-automated one-off that allows you to read the pg_largeobject after enabling security and then export all the metadata as zipped XML files which you can then do a search.

List workbooks containing url actions (postgres query) v9.0.3

What Jeff said.  Here are a few more links that might give you some more ideas.

What's in your workbooks? The TWB Auditor v2.0

TableauFS - File System on Tableau Server RepositoryDataboss

How to use TWB Auditor with TableauFS: Audit Tableau Server files directly -Databoss

Tableau Friction: Need to know your Custom SQL? Use the new Tableau Tool - ExtractCustomSQL.rb

Regards,

Gerardo