Plaster? No, plastic!
My project turned out wonderfully. I installed the medallion tiles and for the perimeter I used a textured wall paper right over the old primed drop ceiling tiles. Like I said, it looks really great. In fact, company that's never seen our ceiling before thought the ceiling was a plaster cast that was actually original to the home (we live in an older house). I left the old existing drop ceiling tiles in place along the outside edge. Where possible, I removed metal brackets that were not part of the ceiling support structure. Then to smooth things out, I used joint compound where metal brackets of the grid touched the old tiles. This way when I put up the wall paper it would not be so obvious where I still had metal brackets. When I feathered out the joint compound between brackets and tiles to my satisfaction, I used a high quality sealer primer (Bin 1-2-3) over all of that. The wall paper was a heavier paper, designed to go over paneling or cracked plaster walls and hide what was beneath it. I don't remember the brand off hand, but I bought that at Menards. For some of my seams, I had to touch things up with a little white caulk, which blended in very nicely. If I look hard or if the lighting is just so, I can see where I have metal brackets behind the wall paper, but it really isn't too noticeable for sure. It might just appear to be a ceiling joist behind plaster to someone who doesn't know - if they can see it at all.