This might not be super helpful as it is a mere suggestion, but you should be able to set the decals you want to be affected by the light to a stencil, which is passed in a post process volume. This in turn will allow you to use the spotlight to shine on a single decal and have it affected by the light.
One of my shader is built with a “generated” spotlight: Elias Wick | Spotlight using a similar method as mentioned by L1z4rD89.
You can map the effects to the screen and affect what you see that way using the masks given. For instance this method I used here is similar and could be used in a similar fashion: Elias Wick | Area