I am still looking into it, but I am somehow thinking that it is not really the definition of epikeia what you are looking for. There is not much more there than I just gave, and it is nothing but an old principle of law. Nothing specific Christian or Catholic.
Here you have Wernz-Vidal, Ius Canonicuм, Volume I: Normae Generales, Rome 1938. This is one of the most commonly used books in the area of Canon Law. In the middle it basically just repeats the definition I already cited.
I also added two pages from "Supplied Jurisdiction according to Canon 209" by Fr. F. Miaskiewicz, Washington 1940 (courtesy of Cristian).