In fact, the imagery clearly shows this path by the Rio Mero continuing all the way to O Burgo/Temple, in fact, being a continuation of the paseo maritimo.Peter Robins said:
Searching for 'rio mero cambre sendero', I find this account with lots of photos http://www.skyscrapercity.com/showthread.php?t=519757 - pages 1-3 are the relevant sections (you might have to wait a while for all the photos to download).
So you can drop down to the shore from Portazgo, follow the esplanade round to the medieval bridge at O Burgo, make short detours to (a) Santiago do Burgo, (b) Sta Maria do Temple, and then continue on this new riverside path to just E of Sigras http://maps.peterrobins.co.uk/google.ht ... 0BT&mark=y where you can take the lanes to the church of Santiago. This would remove a large chunk of road-walking from this route.