I can think of two ways the engine might manage the effect:
1. By sending the entire scene (minus any Spectres) as a texture to a shader which has been applied to each frame of the Spectre(s).
2. Or send a black scene with just the Spectres drawn in greyscale as a texture to a post-processing shader.
Going by what Graf has said in a recent feature request, option 1 won't be possible until the Vulkan renderer is ready. But that doesn't mean it would automatically work either without additional development which option 2 requires as well (but could happen sooner than Vulkan?).
I know it's not the same thing but you could substitute this as a placeholder in your mods until Fill Spectre gets the support it needs. It was my attempt at a sprite version of Fill Spectre so it won't suffer the drawbacks of the purple detection.