di silently succeeds with malformed $DI_ARGS
Boy do I feel stupid. I just realized that my DI_ARGS was missing '-f' before the format specifier string, and that it has been missing for many years. Sorry for the noise and please close.
Thanks for the suggestion, but no, I don't think that will suffice, since I already have M in my $DI_ARGS (sMbuv1t, to be precise). Before I made the edit documented in my second comment above, then recompiled and reinstalled, my listed mountpoints were all truncated to 15 characters. Now they are not. For mountpoints under /run/media/phil/, (that is, those mounted automatically or by the udisk subsystem on Linux), all that was visible in the mountpoint field was precisely /run/media/phil, which...
For anyone else who wants a quick-and-dirty fix for this now, change the hard-coded literal 15 at line 1126 of display.c to whatever you want.
I used an external editor to complete the above and it looks like the final text, which did display correctly in the edit field before I submitted it, was nevertheless truncated where I invoked the external editor, so apologies for that, and I'll stick to in-browser editing now. It should go on to say: "... or the facility to pipe the mountpoint string through an external process such as a shell script before insertion, or both. Thank you."
Make mountpoint field customizable
RFE: More/better notification
New switches: --one-and-done and/or --browse