Burning paper effect problem

I'm thinking one "good" and one "fully burnt" texture. The burnt texture would have alpha high on the edges and low nearer the middle, as "guide" for what parts get burnt first. Then a shader would blend using (originalBurntAlpha-burnLevel)*gradient. Gradient is thickness of the zone between good and burnt..

For example, (alpha-0.5)*5 would turn any original alpha less than 0.5 into 0 (unburnt,) 0.5 to 0.7 would fade from good to burnt, and 0.7 or more fully burnt. Those are final 2-texture lerp values, so 0 means "show good only," 0.2 means show 20% burnt and 1 means "show 100% burnt texture."

That's simple enough if you know fragment shaders (but most plp don't.) Maybe shaderLab can do it. The trick depends on being able to ADD (or subtract) a variable from the alpha. Can't think of a way to do that w/o a custom shader. On a material+any builtin shader, the color is always only a multiplier.