That is trickier since the image itself really needs to have some level of transparency, in addition to the X3Dom object. That’s easy enough to do on the Mathematica side with a command like
imageOut = SetAlphaChannel[imageIn, 0.4]
Using that, I made a first pass at your request by adding opacity to all the stacked hex images:
Technically, that satisfies your request but I’m guessing you want the ability to choose which map is displayed while dimming the others? That would be doable, but it would require a bit more work still. We’d obviously need two images for for each map - a transparent one and an opaque one. In addition, we’d have to fiddle with the so-called z-index of each slice because I don’t believe that X3Dom automatically puts the opaque object on top - that is, transparent objects can sometimes still obscure objects behind.
Also, it’s worth mentioning that we’d probably not be able to do a smooth transition from opaque to transparent without a whole range of corresponding images. I doubt that’s a big deal, though.