[John Arnold]

I think there is some confusion in the terminology here.

Whether you 'plane', 'scrape', 'sand', or 'level', the result is removal of some fingerboard material. I prefer scraping for most material removal, followed by sanding with a hard block.
These days, most of my leveling is done near the body joint to flatten the area after a neck reset. On many vintage Martins and Gibsons, the necks have more taper in thickness than modern necks, and a bit of reduction at the body is less of an issue than it is in the first position.

I also don't see the need to plane the fretboard to correct the relief of a guitar that has an adjustable truss rod.

Adjustable truss rods are a different discussion. In that case, relief is adjusted as closely as possible with the rod, then any minor sanding/shaving/planing can be done to equalize relief on the whole board.

Your method of removing the fingerboard and regluing with an induced back bow makes sense. Of course, there would be a fair amount of finish touchup on the neck afterwards. Do the vintage purists complain about that?

Touchup is minimal, and may not be required at all if there is finish wear on the fingerboard edges. If I do touchup, it consists of drop filling, no overspray. I locate the fingerboard with two 1/2" long brads, drilling the holes for them before removing the fingerboard. That way, the fingerboard goes back in exactly the same place, with no offset edges to contend with. The brads are removed after gluing.
This is not a particularly new technique, just a variation on heat pressing. IMHO, the advantage is that it is more predictable and more reliable.

To be honest, I've never removed a fingerboard on a vintage guitar that was of any value. It sounds a bit scary. Hide glue can sure be stubborn, even with plenty of heat. Do you ever have problems with the mahogany or rosewood splintering and causing cosmetic damage? Any tips to avoid this?

I heat with a heatlamp, and add some water with a syringe if necessary. I use a thin spatula, working from all directions. If I feel resistance, I stop and attack from another angle. I rarely have splintering, but if I do, I peel the splinters off and glue them back in place before reinstalling the fingerboard.

How many extra hours of work do you figure for removing the fingerboard, regluing it, and overspraying (or otherwise touching up) the neck?

About 1 1/2 hours, usually. I spend more time than that on a complete refret.

BTW - How much curvature are we talking about with your fingerboard caul? Could you post a pic of it?

A photo would not be helpful because the curvature is so small (about 0.025"). It is made of oak, and is 1.2" thick. That is stiff enough to cause the neck to conform to the curve when it is securely clamped.