To the best of my knowledge, the solution here is to move your camera far, far away from your scene and use a very, very narrow angle lens to flatten out the image. We have used this technique for rendering out building elevations.
When we render a picture of a train in “isometric view”, you can control by the
rails of the train how “isometric” your view is. When they are perfect behind
the others, your picture looks good. If not you have to move your camera more
away.
While a “true” isometric view optoin for the viewport would be really nice a second trick - at least for stills - is to parent all your scene content to a null and rotate it in the ortho views and make a render there. Sounds awkward - works good. I usually set my view angle to 0.1 or something. The perspective shift is so small that it is absolutely sufficient for an isometric render.