No, that wouldn't work, as you'd have to make this part of a complete global mask. If you know how to do that, that would be best.
What you could try is get the angle/slope of the surface near the center of the map (by hovering over the preview or perhaps rightclick (off my head, so I don't know exactly), and try to project the mask on that angle from a camera hovering over that same center. A bit tricky, I admit, and the edges will still have those outwardly angled clouds I suppose, depending on how big the mask is. And you'd have to figure out the height over the terrain to get the right coverage. I don't know if you can tilt the image map (set on Y) shader with a transform shader (filling in the angle variables somehow), if so, that could be an option.