Camera Based Object Rotation

Is there a way to have an object rotate while it’s looking at every players camera?
For the next sections, Main Avatar would be referring to the player with this avatar I have in mind,
Other Players would refer to everyone else.

Specifically, is there a way to have rules set up so that based on the rotation from the Main Avatar to other players looking at the main avatar, an object would rotate while looking at the other players
If player looking at left side of main avatar, object rotation is 45 degrees to the left,
If player looking at rear side of main avatar, object rotation is 0 degrees
If player looking at right side of main avatar, object rotation is 45 degrees to the right.

I guess in a nutshell, mickey mouse ears style rotation.

I cant make this with a custom scripts cause the vrchat sdk doesn’t allow it,
Im thinking perhaps some kind of constraint, or vertex shader, but i’m more familiar with fragment shaders rather than vertex ones, so any help will be greatly appreciated.

Like Doom sprites?

You want a vertex shader for that. Here is an example of one: Simple Billboard shader for Unity · GitHub

You could rip the vertex shader out of that and use it with whatever fragment/surface shader you want. (Note: I have not personally tested the above shader)