I'm planning to get some red varnish or glyptal enamel to have on hand if I wanted to revarnish worn ballasts. Would putting the varnished ballast in the oven set at 250°f (120 ish °c) be a good alternate to running the ballast in a fixture if a fixture wasn't readily available?

I wouldn't run the ballast at all before it dries out. The liquid varnish does not provide the protection against vibration and mainly evaporates solvent fumes...
I think one of the best methods is to let it dry on  direct sunlight of a hot summer day. But that obviously works only part of the year...

hopefully you are properly vacuuming them while immersed.otherwise you are wasting your time and material.i let a freshly rewound transformer soak overnight in a can.
in the morning i pulled a vacuum on the can and got a huge amount of bubbles.held it for several hours till i didnt see any more.let it drip drain overnight and only then did i bake at 275f a few hours.
it was all done with stinking at that point.so my room didnt smell every time the amp was run.

Yeah I would, would a conveniently sized rubbermaid container and shop vack work?