Space ship shield GFX system

The easiest way to do this would probably be testing the stencil buffer in the shield's shader.

Then, any time the shield is hit, it's just a matter of rendering a mesh that draws to the stencil buffer in the shape that you want to become visible (for finer precision, it's probably possible to use a textured quad, then have the shader use clip() to discard transparent fragments the way Transparent Cutout shaders do). In either case, here's a quick tutorial for rendering to the stencil buffer. This image from the tutorial effectively shows what the stencil buffer can do:

There are more complicated ways of doing it if the stencil buffer is being used for something else, but hopefully this is sufficient.