I am using the orthographic camera preview fairly extensively for making an in-engine Tiling Texture maker. This feature is currently busted since the orthographic camera preview window is rendering static meshes as a flat grey color regardless of what material you put on them.
Changing the camera projection to Perspective fixes this, but that is not a solution since you cannot use a perspective camera to set up views for a tiling texture.
The included Answerhub link has images for this.
Place a camera actor in a level that has some basic geometry of some kind. Observe materials in camera preview window. Then change detail->projection mode-> to Orthographic. Notice all materials get replaced with flat grey.
Head over to the existingAnswerHub thread and let us know what's up.