I'm not sure why a liner is even necessary for a "civilized" (i.e., no primitive, backcountry camping) hike like the Camino?
You won't be fording rivers or pitching a tent in snow.
Stuff sacks (not necessarily waterproof) and an assortment of ziplocks are great for organization, but waterproofing is not really necessary.
In the rain/snow, I walk with either a backpackers poncho or an umbrella. So I guess, if your intention is to walk in inclement weather with a rain jacket and pack cover, where some rain/snow may get between your pack and your back, maybe a liner is good extra protection?
If so, a good choice is a trash-compactor bag for a full bag liner. Cheaper choice than "official" dry sacks, and stronger for the long haul than regular garbage bags.
http://www.amazon.com/dp/B005D50OU8/?tag=casaivar02-20
Yes, you would just leave it in your bag, and organize individual gear groups within it in ziplocks or cloth bags.