I’m afraid that if you don’t have a tenant that can perform search over custom metadata fields when using search engine v2 then you won’t be able to accomplish that in the extension. As noted in some of the links you posted this ability it’s not available to every tenant.
There’s a new version of the search engine that is being introduced where the query by a tenant ID in metadata would be possible (Search for Users), however, this is still not generally available and even if you can already use v3 through the Auth0 Management API, the DAE extension is still making use of v2 (auth0-delegated-administration-extension/users.js at b5e3527f30a41df481d6af03cf78a01d98679beb · auth0-extensions/auth0-delegated-administration-extension · GitHub). In conclusion, there does not seem to be a solution for this at this time.