The chunk of foam that killed Columbia weighed a little under two pounds. That's a pretty small chunk of ice. If it were an inch thick, this would be a piece of ice about 8 inches on a side.

I don't know if ice would or could accumulate after the solids were lit. That's a lot of vibration. Maybe moisture would just keep flaking off as "snow."

You are still left with the not-insignificant problem of somehow removing all that acreage of foam in a few seconds without damaging the Orbiter, getting in the way of the launch, or dropping hundreds of pounds of foam into the solid rocket exhausts, probably to catch fire or leave some kind of scorched mess all over the pad.