Thanks for clarification and great that this is not included in project, but couldn’t someone change the server side code and somehow see more info that goes through?
I know there is that HTML check in https://searx.space/ to see if search interface code is not heavily modified, but on server side anything could go on.
If requests are encrypted in a way that searxng does not see contents then it probably is not trivial to do, but there always is a possibility something clever could be done.
Check out WhoSampled. It’s the largest resource regarding samples/covers.