Blending parts of texture on 3d object with skinned mesh

I have an object with a rage doll effect (I did it my self using the Character Joint since its not a humanoid at all). It uses a skinned mesh of course. Lets say it was a cylinder shape initially, before it became a ragdoll. I also have a fire. I also have two textures, one of it normal, and the other of it burned. What I want to do is, if a part of the object is put in the fire, I want to blend the corresponding part of texture of the object to burnt. This is involves figuring out, what part of the texture the fire is closest to (bear in mind I'm using a skinned mesh) and then settings the pixels near the fire to the burnt texture. Is there a simpler method using shades? How would I go about doing this?