After some coats of new plaster on our old 1920s ceiling, i now have to re-install the AC register, which has four screw holes. Before I took it down it was installed with four two-inch screws right into the plaster, no anchors etc. and since the register is relatively light and doesn't get taken down and re-installed that often, they were holding pretty well.
However now the holes are plastered up so in lining things up i may not hit the exact place again. At any rate there's about an inch of plaster, then wood lathing, likely not a joist above every screwhole. I could just drill a pilot hole and repeat the old setup, hoping a few of the screws connect with the lathing. I could also use plastic anchors, which might facilitate future removal/replacement for duct cleaning etc.