In my experience/view plenty of pilgrims stay in places like municipals when they are not walking towards Santiago. FWIW all the albergues between Santiago and Finisterre would go out of business if you were only allowed to stay in them when walking towards Santiago.
I've met very many pilgrims walking backwards along different caminos, and being allowed to stay at Municipals without any difficulty - @JabbaPapa
has recent experience and might wish to comment on this.
Just in terms of going to Porto, as Doug says above, the route to Fatima from Santiago is known as as pilgrim route anyway so should be no problem at all.
I believe it's highly likely that many other places (including municiplas/parroquials) on different routes (not just to Fatima) will accept you, as long as you are able to explain the pilgimage (route) you are making (and possibly why), and the stamps already in your credencial back up your explanation.
I'm sure there are lots of others who have had useful experiences on this to share.