More things to check for:
1. Toe-in. I've found that Mafac brakes in general seem to need more toe-in than most other brakes. Bend the caliper itself by using a pair of adjustable crescent wrenches. Don't make the mistake of trying to bend the shoe (the thing the pad fits into) because it's only swaged onto the post; it will just break. Once this happens it can't be repaired. With Mafac cantis, you need to bend the post itself near the shoe, by using two small pliers or vice grips (because of the design, you can't really bend the caliper). Careful here too, or you won't be able to get it back into the caliper assembly. 2. Glazed brake pads. Take some rubber off the surface where they meet the rim by using either a bench grinder or metal file or even sandpaper. 3. Clean the rims--if they have any grease or oil on them at all, they will squeek. In a pinch, you can rub dirt on the braking surface of the rim. 4. Check the brake assembly for any loose parts--but do this last. Whatever is causing the pads to squeal is probably also causing the whole caliper to vibrate. This is because what makes a pad squeal is that it grabs, releases, grabs, releases, over and over, many times a second. Like rubbing your thumb on a balloon. Anything loose enough to allow the arms to move forward could be part of the problem-- pivots, mounting bolt are most likely.