My standard for accelerating the mating of the pads to the rim is to force some heavy beltsander belt cloth past the brake pad surface under the exact alignment and direction as when using the brakes while riding. This simulates the first hundred miles of riding in just seconds.

Applying the brake while rotating the wheel forward with the belt moving with the wheel:




Note that Kool-Stop's black compound is their grippiest, so most likely to squeal.
The pink compound is all-weather, with iron oxide and less grippy.

Loose pivots don't actually move with the squeal frequency, since the elasticity of the arms, cable and pads keeps the pivot loaded uni-directionally under braking force. Nearly all of the squeal motion thus occurs in the pad itself and the flexing arm.
Sloppy pivots can affect the effective pad toe however, since the pad will change angle in three dimensions as the pad first hits the rim and all freeplay is taken up by the constant braking force. As such, note that the accelerated bedding-in process pictured above has all slack preloaded out of the pivots in the exact direction as when braking, so the leveling of the toe under force is indeed level with the rim surface.

Additional toe can be quickly applied to the pad surfaces with a stone in a high-speed Dremel tool once the wheel is removed. I would not bend a Campy caliper.

One first thing to check for is whether the toe is equal on both pads, and if not the caliper may need aligning at it's mounting point.

The most common cause of squeal is too much friction, as when the air is humid.

This is how it was explained to me in my youth.

Good point. I'm sure this effect also happens to some degree with the more flexible aluminum caliper brakes, like the formerly ubiquitous Dia Compe 750, for example. The toe in on those will disappear as the pads wear in, but it takes longer than it would on a stiff campy caliper. I was attributing it entirely to less use, but that's not correct, or at least not the whole story.

At any rate I'm glad the OP has solved the issue. Yeah, 80 grit is too coarse for sanding rims. My preference is for 220 or 320P, but it's not super critical.

Thought I’d give an update on this. While sanding the rims and pads certainly improved things, it didn’t take too long for the rims to polish up and start to squeal again. I also noticed there was a lot of shudder through the fork, quite visible and a little disconcerting.

So I took the drastic measure of trimming the pads at an angle to replicate toe in. I started sanding them, but that was taking forever, so I got the hacksaw out and started surgery. They were tricky to cut so finely, but the end result was OK.

I’ve only been for a short ride but early signs are good. The shudder has gone and there’s no squeal, they even feel a bit more solid. So I’ll be interested to see how they go over the next few weeks. I haven’t done the rear yet, but at the moment they seem to be OK.

Handfiles are a good weapon of choice when modding your brake pads.