There’s already a hole for rust protecting the inside of the rocker panels. If you remove the rear tire spat, underneath it there’s a hole for rust protecting. Tire spat has seal in it that will seal the opening. You also need to remove the rear tire to have good access to that hole. Spat is held on by one screw on the bottom of the car. Screw screws into plastic so it opens easily.
From this hole you can push in the rust protecting spay can tube that has multi nozzle at the end.
http://www.tectyl.fi/2009/tectyl_ml.jpg this is the stuff that I used. Condensation on outside of the panel clearly showed that all of that area (and more) that was rusted in MetroMPG:s Prius was protected.