As long as none of it is truly "structural", that plan sounds like a good one. If you can prep everything well enough, POR15 is all the rage these days for protecting metal surfaces.

As for the stuff that converts rust, it is called Rust Reformer and is available in a normal paint can or in a spray can.

On another note, I still find it slightly odd that Saab decided to use a steel subframe for these cars. All of the new-ish Volvos have had aluminum subframes for at least 15 years now.

 

yes por15,you can apply dirrect,or use etching primer.$38 dollar can should do a subframe.

 

On my 1962 Triumph TR4 frame I have cleaned out as much rust as I could using abrasive disks or wire wheels - no substitute for physically removing it. However, there is always some you can't get too, so I then used Eastwood Rust Converter followed by their Rust Encapsulator. Like POR15 this is used by classic car restorers, but unlike POR15 it is not as fussy about needing spotlessly clean surfaces before application. On a subframe, this could be followed by Eastwood Chassis Black. I do not work for Eastwood or have any financial interest in them, but this approach has worked well for me. http://www.eastwood.com/rust-solutions/treatment.html

 

I would just coat it with a nice goopy rust resistant spray, it is not visible so it does not have to be pretty, just spray it and if it is coated, that should stop it for long enough...