A few DIY or home remedies to attack the inside of the tubes is a mix of oxalic acid or trisodium phosphate. Often known in products like TSP. Use it aggressively. For less intrusive and general surface rust removal of the inside of tubes, try full strength white vinegar. When you drain the tubes and see brown liquid, you know its working.
With concerns of ruined external paint and decals, I wouldn't bath the frame. A squirt bottle with small straw that fits the tubing hole is a must.
All tubes are easily accessible to fill and let stand for a day.
The exception is sometimes having a frame with the top tube without any access / call it a port. They can be problematic because of heat cycles and condensation build up through many years. In those cases, I've taken a small drill from the backside and at the seat tube area. Make the hole and fill with your fave solution.
For anti corrosion measures, I use Seafoam products and saturate much as possible, fog the tubes by temporary sealing the holes and openings.