I tried finding the Chans option described by
Brierley in his Camino Portugues guidebook last year. It doesn't exist, certainly not as described. I ended up following a mix of the N-550 and some minor roads until Camino da Gata, and walking west from there across to Camino do Castro where I rejoined the waymarked route into Mos.
Open Street Maps shows a path along the Rio Louro labelled
Senda do Louro Mos. I did see the southern end of this just after Camino da Gata crosses the Rio Louro, but I had already spent some time unsuccessfully trying to find the start point for the path described by
Brierley going north from Camino da Serra da Auga, so I wasn't inclined at that point to undertake any more exploratory excursions.