This is just a PSA regarding an extremely annoying issue I've solved for myself.
In WPF, if you have a Viewport3D and want to export it as an image, you'll probably want to use RenderTargetBitmap. If you do this, and have any solely emissive materials as textures, these will not be visible in the render, and AFAIK there are no sources regarding this on the WWW.
The solution to this problem is simply to add a black diffuse material alongside the emissive material.
Example below.
Hope this helps someone, sometime, somewhere.

materialGroup.Children.Add(new DiffuseMaterial(Brushes.Black));
materialGroup.Children.Add(new EmissiveMaterial(imageBrush));