I would suggest using the camera's top, botton, left, and right members for a full control of the isometric projection.

Other different way would be computing the projection of the quad directly in clip space into the vertex shader. Starting with a fullscreen square (-1:-1/1:1), it only needs to be multiplied by the proportions. Something like this:
Code
outPos.xy = inPos.xy * vImageSize.xy / vViewportSize.xy;
outPos.xy += 2.0f * vOffset.xy / vViewportSize.xy;
outPos.z = 0.5f; // depth into the view fustrum 0<->1
outPos.w = 1.0f;