I do get your drift. Something like that could work, though yes it is a bigger project than I had in mind at this point.
I was thinking of something similar (had discarded the idea, but was thinking of it), to make a frame that goes just inside the window with magnets or velcro, and an opaque, waterproof covering could go on it for showers and be taken off when the shower is not in use.
What is dampcourse?
I think the opaque glass would be fine when the only light is from outside, but not at night when it's dark outside and lights on in the bathroom. Perhaps with the heavy regular shower curtain. I was thinking I could do the fake stained glass treatment, but when the panes are slid out of the way that would be doubled up and wouldn't look as nice, and also would be doubly opaque and let in very little light on that half (the window is 2 feet wide, so it'd become a ~1' wide by ~4' high window when the pane is slid out of the way -- not optimal, but interesting).
I definitely agree that I don't want to plastic things up. I need to be practical too though, we're just not going to rip out a perfectly functional window and put in a new one (or change the placement of this one) any time soon. (We have a bunch of other windows that are not
perfectly functional that would need attention first.)