Wonderful idea, to make a display of half-tones!
Suggest consult an industrial chemist; there is probably an inert white powder, which dusted on, then wiped off, leaves white in the etched-out recesses of the half-tone. I do not suggest using baking soda (NaCo3 ?) but (if my memory is correct) that's what the old-time engravers used; it washed out easily with cold water. Perhaps there is an inert paint which could be poured on, and then wiped? Like intaglio, but white. Do only one plate at first, one which would not matter if lost, leave it for a year or more before venturing to others? What do others think? I am not an expert! Alan.