just make sure the rust is thoroughly dry before applying framesaver. rust is porous and traps moisture (which is why it's so hard to stop the spread of rust). if there is any moisture trapped inside the rust, the framesaver may actually trap it in. since you can't fit the bike in your oven to dry it, the next best thing would be to let the bike sit in the sun on a dry day and get really warm to the touch for awhile before applying framesaver. that way, the rusty areas will at least be free of moisture before being coated by framesaver.

i've used framesaver on all my framesets after being powdercoated. the process of powdercoating essentially removes any protective coatings inside the frame, leaving the internals vulnerable to rust.