I posted that question at https://stackoverflow.com/questions/61792645/how-to-search-for-customer-record-joined-with-instances-of-a-custom-record-which

a) Get the CF field value b) run a search for instances of the custom record that match that value in the specified field. Or is there more complexity to this than I'm realising?

The extra complexity is that I want to do this as a join on an existing customer search.

Unless you have the option of using n/query type requests I don't believe you can. Hopefully someone with more SuiteTalk experience can advise.