Seems like there used to be a thread that listed camping spots on this forum.
Off the top of my head, I know you CAN camp at Orisson, Roncesvalles, Zubiri.
Trinidad de Arre has a nice yard I bet they would let you camp in.
I'd camp at Eunate. I've camped just outside of Cirauqui, on the ROAD (not the Camino) there is a park with tables.
You can camp between Torres del Rio and Viana on the ROAD on the left side, there is a old homestead.
There is a churchyard where you can camp in Ventosa.
::thinking:::
I've camped at Epinosa, San Bol, San Anton,
I think they'd let you camp at St. Nicholas, Boadilla,
There are places near Fromista,
I've camped at Hospital del Orbigo, near El Ganso, I think Tomas would let you camp near Manjarin,
I've slept outdoors at Molinaseca.
I think they'd let you camp at La Faba, Pieros may allow it.
These are some of the places I know of, or places I'd ask.
I think you'd be fine if you just ask and be stealthy, leaving no sign.