I've subsequently investigated the area in more detail, and have discovered an interesting candidate for the so-called 'beddau hir' or long graves of Gwanas.
Aerial Photo of Enclosure Near Lletty Canol
Lletty Canol Enclosure, Shown in Proximity to Both the Brithdir Roman Fort and Gwanas Moor
The best account of this candidate is found on the COFLEIN site (http://www.coflein.gov.uk/en/site/54526/details/lletty-canol-possible-roman-signal-station-above-brithdir), which I will here quote in full:
"A small square earthwork set upon a ridge summit has been identified as a possible Roman military tower. It is set on the crest of a south-facing ridge, commanding extensive views across the upland basin below Pen-y-Brynnfforchog and the course of the Roman road between Caer Gai and Brithdir.
A range of alternative interpretations can be advanced, notably that this is a Roman or early Medieval square ditched barrow, such as are found at Druid beyond Bala (NPRN 404711), and Croes Faen near Tywyn (NPRN 310263). As such it would, with Tomen-y-Mur (NPRN 89420), be a rare surviving earthwork example, most sites being known only from cropmarks. This monument might be compared to the small practice work at Llyn Hiraethllyn (NPRN 89703), otherwise the smallest example of its type known in Wales.
It is a square platform about 5.0m across with a shallow ditch up to 2.8m across on the south-east, 1.1m wide on the north-east and south-west and not discernable on the north-west. The platform has low banks on the north-east and south-west sides. As a Roman work the earthwork has been associated with a road or track passing below the ridge to the south-east (NPRN 91903), suggested as part of the Roman road between Caer Gai and Brithdir (Rigg & Toller 1983, 165; Britannia XXVIII (1997), 399), although this has been disputed as it is a modern feature (Browne 1986) and is depicted on the 1st edition OS 1" map of 1837 (sheet 59 north-east). A tower on this site would command extensive views of the tributary valley to the south-east, but not of the main Wnion valley on the north-west and the Brithdir military settlement (NPRN 95480) may be out of sight. The earthwork is intervisible with the 'Rhyd Sarn' works 11.5km to the north-east towards Bala Lake (NPRN 303162-3)."
The 'low banks' of this monument (if that is what it really is!) nicely answer for the 'long graves' of Gwanas. Caer Oeth and Anoeth would be the Brithdir fort itself. Whether Arthur was thought to have been buried at the fort or at the adjacent funeral monument is not a question we can answer.
There are no other candidates for the beddau hir. Of course, time and the combined ravages of Man and Nature may long since have destroyed any other such monuments in the region.