Interesting to see other Cannondale owners chiming in with the same problem.
Looks like the problem is probably in the fork flex combined with excessively effective brakes then. That would accord with what I feel and see on the bike.
By the way: I don't think this is to do with being 'heavy handed' or bad bike handling. Only the gentlest application of the front brake (i.e. useless in slowing the bike) will allow the issue to be limited to squealing. None of the MTB's with canti brakes or roadbikes with calipers that I've ridden or still ride have had the same problem - found some squaling here and there over the years, but just the usual, nothing like what this Cannondale is doing. This is definitely unusual.
The pads on there now (3rd set from new) are special anti-squeal pads put on by the last bike shop mechanic to try and fix the problem. I've also sanded them in an effort to triple ensure there's no glazed layer on top or any such thing.
It's also not something that happens only when new dirt or moisture gets on the rims. Given that some dry lubricant on the rims fixes the problem, I actually wonder if the mud and crud on cross courses actually helps the brakes to work better by virtue of reducing their effectiveness. Without lubricant of some kind, these brakes grab hard - really hard, almost like glue to the rim. Tried different pads, different amounts of toe-in, different positions on the rim vertically, sanding the pads, applied the Sheldon canti brake setup tips, all the usual.
I can't help but think it's the Cannondale component mix somehow. Perhaps the deep rim on the Gipiemme wheels or the small spoke count allows for problematic flex and the resultant grab/release shudder? I might try out a different front wheel from a friend's roadbike and see if that has any effect. I'd like to think I can keep the carbon fork, the bike feels plenty stiff already without adding an alu fork to the equation.
Otherwise, I guess I'll just keep applying the EzyGlide, it works perfectly so maybe it's not such a stupid solution...