Paul,

I don't think I identified that as the root cause of the problem.

For future reference:

DocumentReference.extension('http://example.org/fhir/StructureDefinition/participation-agreement').value or

"DocumentReference.extension('http://example.org/fhir/StructureDefinition/participation-agreement').value.as(Reference) should work (https://build.fhir.org/searchparameter-example-extension.json.html). But it doesn't (for me).

The only way I got this working is like this:

DocumentReference.extension.where(url='http://example.org/fhir/StructureDefinition/participation-agreement').value

But YMMV.

Best regards, David