Further testing has revealed some sort of blind spot, but I'm not sure where it is; there are entries that can show up in the listbox, but promptly draw up an entry-doesn't-exist error from Journal.pm. I'm guessing this is something I'm doing wrong in handing the entry title over, but I'm at a loss for what. I'm not touching the entry titles.
Maybe it would be better if I scooped up entry IDs and did it by ID, but I can't help but get the feeling I'd somehow screw that up.