Internal access in areas were the tube banks and baffels are located is impractal. Access would require moving large tube banks, and then the inner skin.
Externall access is for the most part un-restricted.
I recommend that. On many repair jobs we have to cut our casing and it never seems to cause any problems. You may not even need to use pumpable refractory.
Are these hotspots that have just developed over time or what is causing them if your liners are all in good shape?