OK, after some more experimentation, the behaviour is as follows:


(1a) Insert > Symbol > Special Characters tab: "No-width optional break" seems to insert Unicode U+200C Zero Width Non-Joiner. This results in e.g. breaking up words invisibly (e.g. the word is now underlined as out of dictionary but there is no other visible indication of the location of the non-joiner, even with non-printing characters turned on) but does NOT seem to result in a line wrap.


(1b) The equivalent num pad code is Alt+8204.


(2a) Insert > Symbol > Symbols tab: U+200B inserts Unicode Zero Width Space. This is what I want, i.e. a line wrap results if this happens close enough to the end of a line, and a non-printing character (square-in-square) becomes visible.


(1b) The equivalent num pad code is Alt+8203.


This resolves the problem for me, since I can achieve the desired result, but is not in line with what can be usually found online. Oh well. I'm recording it here for posteriority