Half-Life 1 was able to do this so it seems silly that I should be forced to do a complex material/mesh switching to just do a simple fade.
You would have to use some kind of translucency somehow. Maybe render to different buffers and then use alpha blending to fade the actor away. You would still have to swap shaders to achieve this effect though. Shader swapping is not hard so just make a fading material that you apply to the actor upon fade begin and then use a timeline to feed the material a value going from 1 to 0,
If for some reason you are set on not using masked materials, you could fake corpse decay effect by moving the corpse underground slowly, with its removal afterwards. However for something decent you best choice would be to swap the material and use a special material with removal effect of your choice.