Well I had rather hoped that somebody would come up with an answer for you here but I'm afraid this looks to be something of a lost cause.
I've consulted M.P.Siddons' Development of Welsh Heraldry which, as far as I am aware, is the most comprehensive documentation of all sources and records of early Welsh Heraldry but he too, could only find the reference in Dynn and and has left the arms of Llewellyn Ddu unidentified. He does point to Bartrum "Welsh Genealogies" as a source for the pedigree but this doesn't seem to be available on-line and, in any case, I suspect, draws on the same source as Meyrick.
The key to the issue maybe the identification of Llewellyn Ddu himself, who I presume is the Llewellyn Ddu ap David "2nd Baron of Endeirnion" of the time of 44 Edward III (approx 1356), referred to in Burke's Landed Gentry under the entry for Hughes of Gwerclas. if this is so then he too would have displayed the arms of Owain Brogyntyn.
Assuming that Llewellyn actually bore Arms, and that this is not a case of "back-dating", I think we are left with the solutions proposed by Meyrick : either Llewellyn was NOT of the direct line and therefore had his own arms or there had been other arms adopted (the Hendre arms he refers to are "Argent, on a chevron gules 3 fleur-de-lys Or").
Unless there is some source that has been overlooked I think we are stuck on this one
Maec
PS I found the following related history which you may find interesting "
www.peoplescollection.wales/content/branas-ucha-–-house-history"