The dye won't hurt anything in the cooling system. You will be emptying the radiator to fix the problems anyway so most of it will be in a bucket on the floor.
Another way to maybe find this leak is to park the van on a clean piece of cardboard, a piece big enough to cover most of the under the motor. Then just leave the van sit there over nite. In the morning you will see a wet spot. Now just get under there and look up and trace where the wet is coming from.