The stone in Bryn Celli Ddu causes problems because it's so very odd. It's so odd that it's easier to ignore it than consider it. There aren't many examples of a free-standing standing stone in a tomb. The only two definites I can think of are Cairn F at Carrowkeel and Cairn L at Loughcrew. Some early reports of Newgrange mentioned a fallen stone in the chamber, but no evidence of this was found during excavation.

Both of the two Irish ones mentioned would have been visible from the entrance, unlike the BCD one, which is hidden away round the corner. They are also square in plan.

If you're right (and I think all of us hope you are), then it's a great bit of spotting. So, while we're considering round sectioned stones what do you think of http://www.themodernantiquarian.com/post/44255 in the same context? There aren't many of them about.