David R. Hansen, Gwen Hinze, and Jennifer Urban have self-archived "Orphan Works and the Search for Rightsholders: Who Participates in a 'Diligent Search' under Present and Proposed Regimes?" in SSRN.
Here's an excerpt:
Regardless of the specific formulation, the search for rightsholders (or conversely, the confirmation that no rightsholder can be located) is an integral component of almost every orphan works proposal. This paper examines in detail the core schemes for identifying rightsholders among the leading orphan works regimes and proposals. Although these schemes differ across many variables, three factors predominate: (1) who is expected to participate in the search process, (2) the nature and extent of the required search generally; and (3) specifically what types of resources, tools, registries or other information-sharing mechanisms are required or allowed.
This paper compares existing proposals' approaches with respect to the first factor: who participates in a search? A subsequent paper will focus on the second and third factors.