When you use a coating like frame saver, some amount of oxygen remains trapped in the coated rust (rust is like an oxygen sponge). That trapped oxygen will continue to oxidize the underlying steel until the oxygen is used up. At that point the rusting would theoretically stop. However, the problem with iron oxide (rust) is that it has lousy structural properties. In other words, it crumbles and/or flakes off (from shock, vibration, temperature changes, etc.). When it does, it takes the coating with it allowing oxygen to once again seep through the remaining rust and further attack the underlying steel.

If you can reduce the thickness of the rust coating (wire brushing, sanding, media blasting, chemical dipping, etc.) and then apply a rust inhibitor, it will work pretty well. Unfortunately, none of those processes (except dipping which is messy/dangerous/expensive) are really feasible inside a bike frame. Using the coatings on an already rusted frame may slow the oxidation process down some, and with repeated applications at regular intervals could even (theoretically) extend the life of the frame. But they'll never stop the rust completely. Given the amount of work involved, you're probably better off to just ride it as is and put your time/effort/money into finding a replacement.

On a new steel frame, it's a bit of a different story. Since the rust is still very thin, the inhibitor is far more effective.