How to update texture during runtime to display damage?

A certain attack emits a bright flash that "sunburns" a character. That is, the pixels in the characters's surface that were touched by the flash should, e.g., turn red.

The first problem is to find out what pixels were touched by the flash. I should be able to find out via a fragment shader associated with a material on the character. The shader could return a constant color. If, at the moment of the attack, I render that material from the player's perspective to a RenderTexture, I should get a texture containing the silhouette of the damaged part of the character.

The real question is, how do I use the silhouette texture to update the character's texture? I need to translate the "on" pixels in the silhouette texture to the appropriate places in the character's texture.

(The silhouette texture only has information about x and y, and I probably also need z. Maybe I could encode z in the color, or use a different shader to render a different texture with information about z).