I also had plantar fasciitis (PF) long before doing the Camino (two years before heading out) - and it flared up while on the Way. Not a fun thing, by any means. Here are a couple of things that might help:
1) Put some good foot/arch supports in all of your shoes now. I bought Superfeet supports, and after awhile I noticed a significant lessening of the pain (again, this was long before the Camino). I have flat feet, so I bought the appropriate type. There are other types for high arches like yours. I still wear mine to this day.
While on the Camino I had a PF flare-up (and it also appeared in my other foot, or at least something like it). I noticed that my Superfeet supports were a bit on the hard side for extended walking, so I switched to a softer and more forgiving athletic model I bought at a Spanish sporting goods store (about 8 euros). In addition, I tried a silicone type I found at a farmacia (about 30 euros). I also augmented the supports with Maxipads, as suggested by the woman who ran the Roncal albergue in Cizur Menor (they were more for blister prevention, but they also provided padding. But they wouldn't stick to the silicone pads, so I used the athletic type the most).
If you want to go all-out, a Scottish pilgrim I met in Rabanal had expensive composite-material foot supports that were custom-made by a foot specialist. He raved about them, and you may want to go that route (he had two pairs, just in case one got lost or broken). I think I'd get some if I ever do the Camino again (I may try the Portuguese route someday).
2) Try support socks: When I had the flare-up, I bought some thin support socks at a farmacia. They were toeless, hose-like socks that gave some support to my feet. They may also help with blister prevention.
3) A standing stretch: While standing, lean towards a wall so both hands are on it like you are pushing the wall away from you. Bring the good leg forward (bend the knee), and extend the other leg with the affected foot behind you. This will allow you to stretch the bad foot (my doctor showed me this technique).
4) A sitting stretch: When you get up in the morning, sit on the side of your bed (or a chair if your bed is too high) so that both feet are resting on the floor. Pick up the affected foot and cross it over your other knee. Massage the bottom of the foot while bending your toes back, thus stretching the foot. As you know by now, it usually hurts the most when you first get up in the morning, so this stretch should help you get going. The article mentioned that the stretch can help speed up the healing time if done consistently.
5) Use pain management: While walking on the Camino, I used analgesic pain gel and ibuprofen to help with the pain. During rest breaks on the trail, I would massage my feet and stretch them in the sitting position mentioned above, and rub more pain gel into them. Here and there along the Camino, there are folks who are skilled at feet (and body) massage - take advantage of that. NOTE: Be careful with
too much pain management - if you "cover up" your pain too much, you may end up hurting yourself worse - see #5 below.
6) Listen to your body: The Camino forces you to become a student of your body. Pain is your body's way of telling you something is wrong. Take the steps to correct it as much as possible: walk slower, carry less weight, take a rest day, change your socks, get better boots, choose a shorter route on the Camino when possible, etc. I did all of these things, because it's no good walking yourself into injury and misery.
Of course, these stretches, massages, etc. can be done any time during the day, or whenever you feel pain. One good thing is that the more you walk (properly), the less your foot/feet will hurt (at least when it comes to plantar fasciitis). Of course, the main pain will come in the morning when you are getting out of bed. With that in mind, you'll want to grab a bottom bunk bed at the albergues - getting down from a high bunk bed with plantar fasciitis can be painfully difficult, and may even cause you to hurt yourself more.
Anyway, I hope these tips help (check with a foot specialist for the most accurate medical advice, though). I managed to walk from St. Jean to Santiago with my funky flat feet and plantar fasciitis, so with the proper training and maintenance you can too.
