Well, yes; my point about musicologists really related to structure rather than content, but I wasn't very clear about it. Eng. Lit. (however early) won't necessarily get you too far, either, as it hadn't been invented when the (putative) Arthur was around, though the approach may not have been all that different. If you're not too fussy about authenticity -you can't afford to be, if you're going to write a novel longer than about 3 words- you might like to play around with Anglo Saxon narrative verse forms; not so very different, so far as can be told, from some of the early Welsh, and quite easy to write once you've got the hang of it. For subject matter, yes, perhaps parts of the Mabinogion; but again, that was written down much later. On the whole, I'd avoid Irish texts for Arthurian subjects; and don't believe a word you read in Robert Graves unless you're desperate!