Interesting post, PhysioIan.
I think you're right, and having read lots of stuff and tried a range of things I think I'm now a lot closer to working out the underlying problem for me.
First, it came on after I'd resumed running too quickly after an ankle sprain. Tight calves/ankles are always an issue for me and my whole lower leg had even less flexibility after the injury. This put more strain on the foot structure. When I upped the pace on tarmac (a road 10k) it gave up.
After an initial period of bringing down inflammation, supporting it with an insole and a few weeks of rest, the key therapy was stretching - the type you mention being very important, but also whole foot stretches. I felt that direct work on the injured area needed to be complemented by work to increase flexibilty and strength in the surrounding tissues and structures. Balance work helped. Mountain biking on singletrack was unexpectedly beneficial in the later stages, with long sections up on my toes, continually adjusting weight bearing.
Over the last few days I've found that I can run OK off-road, but tarmac triggers recurrence. I have a new theory that the cushioning in my road shoes isn't good enough, and am off to buy a new pair tomorrow. I've partly tested the theory already though - I was talking to an O/70 runner before the West Yorks five mile race yesterday and he commented that his physio had told him to throw away his shoes (which were the same model as mine). This got me thinking and I immediately went and changed into another pair of shoes - my gym shoes - which I happened to have in the car and which have heavier duty cushioning. I also added a sorbothane layer under the insole.
I managed the race without aggravating the plantar fasciitis at all.
So I'm hoping I've got it cracked this time, but as you say, it's something of a detective exercise. I don't think orthotics are always necessary, or that the problem is inevitably one of gait or anything else.
I've just managed 3 races in 4 days, and although I'm aware that the pf hasn't completely gone away, it's feeling better than it did last week. My fingers are still crossed though, and I may not have escaped from this thread yet!