Quite apart from the England-Wales factor, I doubt Hereford is a big enough destination for trains from Brecon and there's nothing really big in that direction until you get to the West Midlands. Seeing as Cardiff only justifies two or three coaches an hour from Birmingham it's unlikely s north-eastward route from Brecon would wash its face in any way.
It seems to me any rail service would have to serve Cardiff directly or by an easy connection with a good average speed, which suggests a totally new line to Abergavenny rather than trying to reinstate any of the disused ones. It's a shame we aren't allowed to convert canals into railways any more!
But seriously, with a population of less than 10,000 at Brecon and a few thousand more in the villages in between, and the whole area within a National Park, it's not going to happen.