Attach receptacle box to baseboard via screws through plaster ears?
The holes in the plaster ears of the box will take screws very well. That is what usually holds the box in place when it is mounted in a cabinet.

To allow the cover plate to set well, you could use 4 gauge screws or a larger diameter and countersink the holes in the mounting ears a bit to help let the receptacle set tight to the face of the box. You would do well to break off the ears on the receptacle at the score lines to help clear the screw heads too.