I've got a steel cyclocross frame I've ridden for a couple seasons. I didn't get around to treating it with Framesaver before that, and after a couple of race seasons there's definitely some surface rust inside the frame tubes. I plan to treat the frame with Framesaver while I have it apart. I'll also be drilling a drain hole in the bottom of the BB shell. The most easily reachable areas (BB shell, seat tube) I plan on cleaning with a wire brush, but there's some rust on the surface in other places, too. Is it necessary to treat and/or remove this rust before applying Framesaver? If so, how?

...
...I've never found it necessary in cases of light surface rust like you describe. It works by coating whatever surface you present to it and cutting off the oxygen. I have done a few frames that were quite prodigiously rusted internally, and on those, one I sealed up all the holes and the BB and filled it with the non-toxic stuff, Evapo-Rust. Which is kinda expensive, but safe and reusable. And the stopping up all the holes business is a PIA. But that works.

The other few I just soaked in a large container (frame and fork sized...kiddie swim pools work for this) of Oxalic acid, which is somewhat toxic and is absorbed through the skin and concentrates in the kidneys. So wear golves and do it outdoors. It's a PIA neutralizing the acid before disposal with large quantities of baking soda or something else basic.

So for light surface rust this is overkill.

Rust is a product of oxidation of iron; stop the oxidation and the rust stops. It in not like cancer cells where it all has to be removed to stop its spread. While it is helpful to remove as much as possible in order to get an unbroken inhibitor coating, putting the coating over some existing rust will work just fine. Get as much off as reasonably practical, coat with inhibitor and ride.