I even cut out old plugs from broken boards to do that same repair. I screw the missing plug onto the fin first, tape off everything real good ,fill the hole like you plan and then set the fin into the existing plug to hold everything in place. then sand and glass just like you said.
Howzit moai, First get the old resin mix out of the hole and flush it with fresh water and let dry completely. Then cut a piece of foam to fit in the hole and set it in with resin /Q-sel mix. Sand and glass the area and hotcoat. Now just do an install like it was a new board. Doing it the way you want will work but I've seen the glass delam over the plug when cutting the tape that has to be put over the plug slot.Aloha,Kokua