Is it too late for frame saver on a bike that has already been sloshing its way through winter? Is it possible to somehow still "save" my frame?

It's never too late for Framesaver. Pump your frame full of it.

Salt under framesaver is still salt, plus it's very important that the metal you're coating inside be completely dry or Framesaver doesn't work properly. I've taken used frames or conversions and literally flushed them with a cleaner/degreaser. Then I've put them right next to a space heater that heats them up to where they're quite warm (don't go burning your decals off, but make it hot enough so it feels like it's parking in the sun on a hot summer day). Don't squirt cleaner where you won't get it out again, like blind holes, and such, but get the bottom bracket, three main tubes, fork steer, and head tube. It's the down tube and seat tube that usually get the most corrosion, plus your perspiration can get your fork steer tube corroded as well. Flip the frame upside down and sideways from time to time so no cleaner got to pool anywhere. I stick a q-tip into the holes in the head tube and bottom bracket and angle them into corners on the main tubes to be sure there's no lingering moisture. Anyway, when the frame's had a day or two of nice warmth and everything looks and feels dry. then use a whole bottle on the frame. It works better to do two or three coatings, allowing it to dry in between (otherwise you just make a big mess and the stuff just pools in the bottom of your frame). Clean off anything that drips out with mineral spirits, and you're ready to go. It'll last for years.

You can also use LPS #3 Heavy Duty Rust Inhibitor instead of frame saver its basically the same thing. McMaster-Carr sells it for about $8 plus shipping.

I just pour motor oil into my frame tubes and slosh it around a bit.