The following are dissertations that are publicly available online on non-password protected sites. If you know of other dissertations that are readily accessible, please let me know. I will add a permanent link to this page under Resources.
British dissertations can be found at EThOS (Electronic Theses Online Service). Some dissertations are ready for download, while you may have to wait several months for delivery of other dissertations.
Allen, David Mark. "'Deuteronomic Re-Presentation in a Word of Exhortation': An Assessment of the Paraenetic Function of Deuteronomy in the Letter to the Hebrews." Ph.D. diss., University of Edinburgh, 2007.
Chivington, Ryan D. "An Investigation for Possible Parallels of the Roman Imperial Cult (Caesar-Nero) in the New Testament Book of Hebrews." M.Th. thesis, University of Pretoria, 2006.
Heemstra, Marius. "How Rome's Administration of the Fiscus Judaicus Accelerated the Parting of the Ways between Judaism and Christianity: Rereading 1 Peter, Revelation, the Letter to the Hebrews, and the Gospel of John in their Roman and Jewish Contexts." Ph.D. diss., Rijksuniversiteit Groningen, 2009.
Isaak, Jonathan M. "Situating the 'Letter to the Hebrews' in Early Christian History." Ph.D. diss., McGill University, 1999.
Kim, Daewon. "Perseverance in Hebrews." Ph.D. diss., University of Pretoria, 1995.
Manahan, Ronald E. "A Re-examination of the Cultural Mandate: An Analysis and Evaluation of the Dominion Materials." Th.D. diss., Grace Theological Seminary, 1982. [Contains a section on Hebrews 2:5-9]
Whitlark, Jason A. "Fidelity to God: Perseverance in Hebrews in Light of the Reciprocity Systems of the Ancient Mediterranean World." Ph.D. diss., Baylor University, 2006.