>> The directionality characters would be highlighted, that's all (and it 
>> would be easy to add a toggle command to (un)highlight them on demand, 
>> see attached).  This functionality is orthogonal to the one you propose 
>> AFAIU, both could coexist.
>
> It could, but I don't think we should install such features.
>

Why not?  Is this not simply an improved (and built-in) version of your 
initial suggestion to use glyphless-char-display?  Except that 
glyphless-char-display is global (whereas buffer-display-table is 
buffer-local), and that a tofu is not as visible as a character 
highlighted with the font-lock-warning-face.

Here's an improved patch which also highlights these characters in strings 
and comments.