Hi Andras,
I took a tent with me when i walked the Norte from Santander, I used it 3 times, 2 at campsites and once outside an albergue(Soto de Luina). I reckon there is the possibility of pitching outside a few hostels, if the hospitaleros agree,Santillana del Mar, San Vincente de la Barquera, Serdio, San Esteban, La Isla, Sebrayo, Camping Deva, Soto de Luina, Cadavedo, Pinera, Tapia, Ribadeo,Gondan, Vilanova de Lourenza,Baamonde, Miraz, Sobrado. I have picked these because they have grass areas outside of them usually part of the hostel, i have a feeling some may, some may not, Sobrado will, they did for a few people with dogs when i was there, and possibly La Isla.There is a few i am not sure about, i didnt see them, Luarca, Tol, Pineres, Boo, Polanco, Cobreces, Comillas, La Caridad, Mondonedo, so there could be more possibilities.
As to camping(or sleeping) wild i suggest you meet some young Spanish people, i met a lot of people who did sleep wild on the Norte, all young Spanish. Walking the Norte can be a bit of an adventure, a lot of the Spanish treat it as such.
Buen Camino
Mike