That situation has happened to me before. I was in a training session with a woman in october and we started talking about writing. After a while I got up the nerve to ask if she was doing NaNo that year and she kind of looked at me sideways and said yes, she was. And another woman started asking her what kind of things she wrote and from the answers she definitely had an LJ. I wasn't sure of the etiquette for that situation so we ended up kind of standing around in silence for a while.

TL;DR - We def. need a pan-fandom comm to deal with things like that. Or an agreed upon hand signal.