gl_VertexID barycentric coords

One way to generate barycentric coordinates from a vertex stream is to use gl_VertexID. This requires that your vertices are ordered in the vertex buffer in a regular fashion as gl_VertexID is the vertex position in the buffer, so vertex optimization using an index buffer will likely break this.

Step 1 is to generate a vec3 in the vertex shader:

//VS

out vec3 ex_BarycentricCoords;

void main(void)
{
    //your usual code
	vec3 bary;
	int i = gl_VertexID % 3;
	bary.x = float(i == 0);
	bary.y = float(i == 1);
	bary.z = float(i == 2);
	ex_BarycentricCoords = bary;
}

Step 2 is to use these to get outlining in the fs:

//FS

in vex3   ex_BarycentricCoords;
out vec4  fragColor;
void main(void)
{
	vec3 line = 10.0 * (0.1 - clamp( ex_BarycentricCoords, 0.0, 0.1 ));
    fragColor = vec4(line,1.0);
}

Step 3 would be to get anti-aliasing by using derivatives (to come...).