rubyslippers said:
The original Pilgrims DID carry everything with them,
According to my research, early pilgrims carried only a small trapezoidal leather bag, flat with no closures. It had symbolic meaning, it was open to remind the pilgrim to give and receive freely, flat to remind them to depend on God and bring little for the journey, and made of skin to remind them of the mortification of the flesh. They sewed coins into their robes, carried water in a gourd, and a few personal items in their bag. Some food was purchased along the way, and religious establishments had certain obligations to provide a certain amount of bread to pilgrims. Perhaps they hunted along the way and surely gathered berries and edible plants.
Another issue with a completely self-contained trip is, where to go to the bathroom? There are few public bathrooms and most are in bars or albergues that wouldn't be very polite to use without buying anything. Of course one can go in nature, carefully following "leave no trace" principles, but there's not a lot of privacy especially around urban areas.
Regarding the legality of "free camping," it depends on the region and the area. Generally it is illegal to camp within a certain distance from towns, historical sites, established campgrounds, water sources and roads. Private land of course is also technically illegal but if you are discrete off in a field somewhere you will likely have no problems. I believe camp fires are illegal except in special parts.
I love to go on "real" self-contained backpacking trips, but I just don't think the camino is well-suited for this. There are hundreds of other lovely hiking trails such as the GR11 in the Pyrenees and the Picos de Europa that would be ideal for a self-contained trip.