I'll try to implement it. Meanwhile, what you probably need should be relatively easy to achieve in an image editor. For example in Photoshop just change the blending mode of the map layer to Multiply.
i thought about that but I'm trying to import the image in wonderdraft so i need to go in photoshop to make it an alpha first which is never 100% clean and it really bugs me out. No worries if it's not really useful to you, i will continue using my method :)