JB, you should list the books you are reading in comments; not sure if anyone else would be interested but I certainly am.
I am using books by Brian Tierney and Annabel Brett; Richard Tuck and Quentin Skinner have written on natural rights. I believe Ian Shapiro's book is written from a Straussian perspective. Tierney and Finnis are critical of Tuck's book, so I am not using it--I need to read the primary sources for myself. I have not had a chance to look through Skinner. His books were put on reserve for some courses.
I have not come across anything that provides complete medieval texts filled with the use of ius as subjective right--it is not clear to me how standard this usage was in canon or civil law.
From the Journal of International Human Rights:
Brian Tierney, "The Idea of Natural Rights--Origin and Persistence"
Michael Zuckert, "Natural Rights and Modern Constitutionalism"
Mary Ann Glendon, "The Rule of Law in the Universal Declaration of Human Rights " html, pdf
OLL section on 17th ce theorists
Michael Zuckert's faculty webpage
Christopher Wolfe reviews Michael Zuckert's The Natural Rights Republic
Page for his Natural Rights and the New Republicanism
Page for his Launching Liberalism: On Lockean Political Philosophy
"James Madison," "Thinkin' about Lincoln," "Natural Law Without God? Considering Russell Hittinger's The First Grace"
Audio files of his lecture on honor; info on Prof. Zuckert's Riley lecture
"Theorists Smith and Zuckert Argue Nature of Liberal Tradition"
Evidently both Prof. Zuckert and his wife were students of Strauss.
His stuff over at Claremont Institute: 1, 2
Vindicating the Founders website (author's page)
To be looked up:
"The ius commune and its demise"
"The Medieval Canon Law of Contract and Early Modern Treaty Law"