No, none of
us have EVER gotten excited about something and had that happen.
Seriously, it's something that still worries me. I mean, Jim's got about ten books left in this series, which means there are a lot of ideas in his head that haven't reached paper or computer yet. And even the most well-meaning fanfic writer can get overwhelmed with "But *I* had that idea! He must've gotten it when I posted it online!" sometimes (we've heard horror stories). And the ones who have that moment of convergence but never act on it will likely be bothered by it for the rest of their lives. Jim's a fan himself, so he really would wish that nobody ever feel like that.
So, yeah, fanfic's okay around here... but out of respect to those awkwardnesses, it might be better to not post it openly, to say that it's there and available, and DM/email it to those interested. That way, everyone who wants to can read it, and there aren't any of those aforementioned possible booby traps. NB: This is NOT a directive pointed at anyone in any way whatsoever. It's just a possible route, that's all.