The current models for “Record Locator Services” does get down to using DocumentReference. But that is just the current definition of “record”. The location services are around finding communities that have records on a specific subject. That functionality is not restricted to DocumentReference.
The main method today is to use IHE-XCPD to send out federated patient identity lookups. Those with records respond with their definition of the patient in question. The originating org can then determine how well that patient definition fits, and now it has a “location” to address further requests toward.
There is not yet a FHIR specific solution for this kind of nationwide federated identity query. Mostly because the problem has not been brought forward to IHE.
Further, I need to note that DocumentReference is not as limiting as many people presume it is. The ‘document’ in DocumentReference simply is defined as a set of bytes that have a mime-type. This could be a CDA document, but could be a FHIR-Document, or even a FHIR searchset Bundle. The limits are not fundimental to the DocumentReference, they are fundimental to the governing community that agrees to the kinds of information communicated. To this point, there are some communities that have defined things like the current medications in FHIR MedicationStatement form, which just returns a FHIR Bundle of MedicationStatement resources.