They already have, actually... the Variant Editor in the Assembly Kit has an environment file associated with it's preview window.
You can replace the default environment file at ..\assembly_kit\editor_working_data\Weather\
variant_editor_day.environment with any environment file you wish... just keep the default file name.
Also, GEM can be installed to the Assembly Kit binaries folder if you want to view those effects as well.
The shortcoming is that the scene in the editor window is pretty barebones. You do get to see the lighting on the unit though, along with a skybox and some water. There may (or not
) be a way to add terrain to the scene, but this is already quite helpful as the Variant Editor loads relatively fast and you can get predictable lighting each time... presuming there isn't some difference in how the lighting is rendered between the game and the editor.
Still playing around with it...
edit: Normally, when you rotate a unit in the VE, the whole scene rotates. However, holding ctrl while left click dragging will rotate the scene (and thus, the light source) around the unit. So you can get a preview from different angles. For taking screenshots (using sweetFX/GEM) the editor's preview window must be maximized (hide the info panels) in order to get the correct aspect ratio.