Change Shademode to get Data from one provoking vertex

I really need the good old Shadermode = Flat in Unity in order to reduce my vertexcount by a factor of 3.
Is there any way (no dx11 pls) to get data from a single provoking vertex?

Some information about what I am talking about:
Nvidia.com Shademode
Flat Shading explanation

Thank you!

The closer I got to, which doesn’t work on mac is

Shader "Flattener" {
	SubShader {
		Pass {
			GLSLPROGRAM

			#extension GL_EXT_gpu_shader4 : require
			flat varying vec4 color;

			#ifdef VERTEX
			void main()
			{
				color = gl_Color;
				gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
			}
			#endif

			#ifdef FRAGMENT
			void main()
			{
				gl_FragColor = color; // set the output fragment color
			}
			#endif

			ENDGLSL
		}
	}
}