not tried that kind of repair but the rust reformer would work great for it, maybe a long handleld thin brush that would fit in? any way you can pour some in with a small funnel?

Wow...guess I am a little late for you guys, but somebody else might be thinking about doing this. I did make an effort to arrest rust inside my frame as follows. With the body off, I put the frame on a rotisserie I made from oak pallets and fabricated brackets to mount the frame. I carefully ground the body mount cage to frame welds off the frame to get access to the holes underneith. Do this carefully so you can reweld them back on when done. I spent quite a lot of time wraping the frame with a mallet to shake any rust inside loose. I then used air pressure and vacuum pressure to remove as much scale and debris as possible. Don't be surprised to find nesting material and whatever could be stuck in the holes. I was sure not to rush this job. Next, I made a wand that would spray undercoating material with pressure from my compressor. My wand was a converted siphon washer from Whitney for about $8. Eastwood sells an undercoating product similar to what I chose and that leaves a soft, pliable film over metal which prevents rust. Zebart also uses a similar rust preventive coating. I put the want inside all frame holes and sprayed a liberal amount inside and turned the frame to promote coating. I did this until the material ran out or I was confident everything was coated inside. The body mount cages with nuts were welded back on when finished. The material I used was black, so it is barely noticable inside the frame. I would recommend this process to anyone who is doing a body off rebuild. If you need to do any welding on the frame afterward, the immediate coating will vaporize leaving the area unprotected. Rust never sleeps!