By the way, on the way up to Caer Bach we came across something odd beside the track: looks like a chamber of some sort (although very small internally) underneath a massive flat boulder. It looks very man-made, at least the space underneath the "cap" appears to be stone-lined. OS ref is (very) approx SH743727 but there's nothing on Coflein at all, or in the Frances Lynch book Blossom has, that covers the area in some detail.
Have you come across it on your visits?