I misread the original post.
If the screw heads are totally exposed and not recessed, Ed's suggestion is what I would suggest too. You'll need to prepare the corrector plate covering it very well to protect it from grit and dust that will be created. A little drop of WD-40 will also help loosen the screws, but it can leave a little residual stain if not wiped off afterwards.
If the heads are recessed, you will need to use the Dremel in another way. You will need to use a tungsten carbide 'fissure bur' (see picture) to cut the slot into the head. A steel bur will have no significant cutting capability on these stainless (?) steel screw heads. Again, you will need to protect the corrector plate. Will need a steady hand to control the Dremel.
