Creating one-way mirror-like lenses
If you put clear tape on frosted glass it becomes see-through. 
You could try that if it's meant to be a momentary peek-though situation.



Or you could mostly frost it with white alcohol ink. 
Just leaving enough of an open lattice to achieve a veiled appearance.

I'd think a decent way to accomplish that would be to 
put on some gloves, wad up a small piece of fabric, 
wet it with the alcohol ink, then dab it on the glass. 
Practice on a scrap piece of clear plastic 
a few times to get the technique down, 
like from a 2-liter bottle that you cut apart.

I remember hearing about see-through concrete that had similar properties: link to article