Sorry for the delay in replying.
I don't believe the Plantard narrative, as you put it, ever intended this, and the notion that it did was another of the errors in the Howells book on the PdS (as I think you yourself highlighted on the thread about that book). Plantard's intention was merely to establish a familial connection back to Merovingian times, so the idea that his supposed bloodline might date back even further, and be even more illustrious, was something of a bonus to his project, I guess.
According to Howells (perhaps we should start calling him Howler) Plantard actually told BLL about the Jesus bloodline. However, AFAIK, it was the other way round! HOWEVER it is, IMHO, very important to remember that Plantard did NOT just try to merely establish a familial connection to Merovingian times. Remember that the Priory Documents also try to create a historical pedigree for the Merovingians that claims an ancient Jewish connection of a particular kind. IMHO this distinction is key.
My recollection of the way that played out was that the HBHG guys - either when they interviewed him as part of the third Chronicle film, Shadow of the Templars, or else subsequently, when doing interviews for The Messianic Legacy, my memory is vague on that - put to him the idea that this might be about a sacred, holy bloodline, dating back to Biblical times, to which he gave a very non-commital answer, along the lines of, "Well, this was all a very long time ago, one can't really be sure, but who knows, etc."
Off the top of my head, that's nearly right, except that Plantard made sure to add something that reminded the observer of the Jewish nature of any supposed bloodline. He said something along the lines of - 'Well, this was all a very long time ago, one can't really be sure, but it is important to remember that Jesus had brothers' - I think that was what was said (again I stand to be corrected).
So, my personal view on that would be that whilst the HBHG authors may not have been the first to argue the notion of a holy bloodline, they were the first to do so in respect of the mystery of Rennes and Plantard's Priory, and that connection, again in my view, was a completely mistaken conflation between material released by the PdS pertaining to a bloodline, and rumours about a significant religious burial in the vicinity of RLC.
So no, I don't believe that was ever intended to be part of Plantard's narrative, and he probably couldn't believe his good luck when other people suggested to him, and subsequently to the wider world, that it was.
I agree, IMHO the Jesus bloodline was never intended to be part of Plantard's narrative. However, I don't believe that Plantard was at all pleased with the way his narrative was hijack by the 'bloodliners'. In the end he completely changed the manufactured 'pedigree' of the Priory of Sion to, IMHO, distance himself from the 'bloodline' claims!