I've been modifying some pwbs that were deficient in ground vias and developed a decent process.
Drill holes with .7mm drill using high speed drill press. I have one made for drilling boards though a dremel in a drill press holder works well.
Clean off solder mask on both sides with a dremel and 1/8" diam. ball grinder. Dremel is runing from a variac which drops voltage down to 36V so the speed/torque are way down. Holding the tool almost vertical to the board is least aggressive cutting. Move the tool in a small circular motion. Fairly easy to clean a "pad" on top of the ground plane around the drilled hole and not destroy the copper.
Solder a #26 bus wire through hole top and bottom.
Next time, will be more careful when releasing a board.