Hm. Welsh material is not really my area of expertise, but the only text I recall featuring Carnwenhau is Culhwch ac Olwen. Do you have an example of one of these 'frequent mentions'? Carnwenhau did not enter the foreign Arthurian tradition as Caledfwlch did as far as I am aware, so I presume this would necessarily be a Welsh motif, but I've not seen it.