Actually, I just heard back from my Adobe contact named Susan Vertrees,
and she said:"Yes, you can now set negative events. I don't know if it
is in the documentation yet, but I have another client who has been
doing it for about 18 months now." There you have it from an official
Adobe consultant. Until they update the official documentation, I'll use
this as my official answer for now. Thanks for chiming in, though!