I had Kreem do this in my old SR500, with the tank rusting again. I was able to get ahold of some of it with pliers and pull it out. From there, I took the tank to a radiator/fuel tank repair shop that "boiled out" the tank and recoated it with the RedKote hard shell liner they use, along with soldering up some holes.
The one thing they didn't guarantee was that the paint could be damaged, so you should be ready for a repaint if necessary. They did tanks for the dealership where I worked and there was no problem with paint damage, but they would not guarantee it.
KLX650C, Zephyr 550
SR500, Bultaco Sherpa T