Hi Iolo.
I presume you know about the regular (most years) Marconi Day special events featuring the Waunfawr site.
What you can do from Cefn Du is a WAB trig point activation. It’s TP2075.
To use it for SOTA you’d have to take inspiration from: ¼ Kilometer Wire Antenna - Thats not an antenna, THIS is an antenna! 
activating from Moel Eilio. But I make it about 2½km, so that would be outdoing even Marconi! 
I must go exploring there some time. I wonder if there are any traces of the narrow guage railway to the masts. Perhaps it was just for construction and all is long gone. Also the ROC site down the lane.