I have, in reverse; considering how wet it's been lately, I'd bet it'll be pretty boggy between the river and the ground beneath (east of) the ridge and the river crossing (if you intend to) might be a bit "interesting" I came from somewhere near Fan fechan, more or less heading east and even in fairly dry conditions it was definitely a good idea to wear gaiters and the river was quite fast flowing.
Using streetview, it looks like the farmers take a quad up the southern side of the Nant y llyn - that might be a good place to start, assuming the river's not a raging torrent.
Don't forget to do a trip report