I wouldn't say "just not possible," but I suspect that Poser's too limited to do it. Flexibility isn't part of its design goals.

I'm fairly sure that Blender permits rendering an isometric projection, for instance. It's possible in 3ds Max, but it's difficult to get the exact rotations since you can't actually select the viewport cameras. Maya makes it pretty easy, since it treats the ortho cameras just like the perspective cam, and you can even select them in the viewport.

As I look at the approach you've been taking, it looks like you're trying to rotate the objects to get the result you want, when what you need to be rotating is the camera. Lay out your scene/map in a physically accurate manner first, without regard to viewpoint. Then you can start looking at where to place your camera and how to configure it for the result you want.

An isometric view eliminates all perspective distortion—the stuff in the back is just as large as the stuff in the front. A camera does not naturally behave this way, but the longer the focal length (higher zoom level), the more perspective gets flattened out. So while you may not be able to get a true isometric render, you can sometimes get something that's close enough, even if you are limited to using a perspective camera. Of course, this depends greatly on the limits of the camera you have. I don't know what the upper limits on Poser's cameras are, but you should be able to at least simulate the longest practical lens on the market, which is 1200mm (they cost US$120,000, if you're wondering).