@Khas
Thank you for your suggestion. I'm doing parallax mapping with pictures and for it I'm using Galv's Layer Graphics as well as TDD's Bind Picture to map, which haven't been updated since 2015-11-14! That makes me believe TDD's plugin might not be perfectly compatible with MV's current version or other plugins anymore.

It's somewhat working for me ATM though, and sadly there are no alternative plugins for binding pictures to the map.
Funny enough, your plugin plays nice with TDD's old plugin and if I place a picture on the "top" layer it will actually display above your fog!
Galv's Layer Graphics does not work well at all though. If I want the fog above everything it works, but only if I place your plugin above Galv's. If I place his plugin above yours, HIS plugin stops working and don't display anything. Quite odd since you would assume the opposite would happen if there was a problem, right? Well, that incompatibility might be on Galv? I don't know.
I guess I could change my parallax maps to only use TDD's Bind Picture to Map and ditch Galv's plugin. But, deleting and binding pictures on each transfer is much more inconvenient than just adding a tag in the map's note section. Also it's a little scary to heavily depend on a plugin that have not been updated in such a long time... (and has probably been abandoned since there isn't much activity in the forum topic either)
EDIT; Actually, I know you're not working on your plugins anymore but IF you'd ever come back to them. Would a plugin setting for choosing a Z layer for the fog be an easy implementation and something you would consider? It's about the only thing I can think of that would make this plugin even more awesome.