If it uses a good toner material, and not a dye, opacity could be okay. I'm going to look into them.
BTW, needed to make a simple black on clear decal last night (for my trademark brass nameplates). Sure was easy using the laser printer and laser decal paper- one heavy coat of Testors Glosscoat, and when that coat was dry the decal was ready to use. I always print several copies on same paper- glad I did, I ruined the first one, but the second went on fine (I had printed three).