You can pour the oxalic acid solution into the tubes after sealing up weep holes, bottom bracket, etc. It has been suggested before that very large garbage bags could act as the container for the frame. I'd try this outside or in the tub in case of leaks. Double or triple up the bags for added security.

The only shop I know that would derust the inside of a frame would be a plating shop. They could soak it in the alkaline bath. But it would have to have the paint removed and cleaned first before they'd do it.