hi Chaps i did wales N to S about 10yrs ago in my 90 i used a combination of Glyndwrs way, sarn helen & nant y moch then onto strata florida with lots of ohers going east west thrown in to make a long weekends green laning.
I used to be the dyfed/powys ROW rep for the green lane association(GLASS) so had lots of lane info, unfortunately sarn helen is closed except for the section between aberdulais and banwen now under TRO restriction(much political wrangling going on at mo with the owners of walters arena that want further restrictions on the byway passing through it)
simon will know about that no doubt as he uses the site.
many other tracks in wales have suffered the same fate due to the NERC act as most N/S routes were RUPPS
I know friends in a local MC trail club that are using sarn helen, however i also have another friend in the s.wales police off road team that are looking to make examples of pple they catch on sarn.
BE AWARE the river crossing is a favorite spot to put the bike down and lie down for a "nap", if you stop to see if he's ok you get knabbed with no need to chase.! that came from the horses mouth.
so... it can be done but mainly on private land the old long routes are gone, i hear the forrestry commision are looking into sustainable routes for bikes in wales.