Bruce O.: A good reference (so I hear) for the medieval Welsh genealogies is called Early Welsh Genealogical Tracts, Cardiff, 1960, ed. A. W. Wade-Evans and P. C. Bartrum. I haven't managed to get this book on ILL yet, though; I only know of it from works that cite it. Like you, I haven't yet got hold of Rachel Bromwich's Trioedd Ynys Prydein, either.

T.