Thank you for all the replies, everyone!
Just wanted to confirm that yes, this is the same venue as always--they didn't pick a different one on me. I actually didn't take the menu into consideration at all when I made my choice, thinking it was just going to be cocktails at the bar and there wouldn't be a meal involved (maybe appetizers for anyone who wanted something to eat). I did take a look at the menu when I first heard about the change, and while they could potentially make me, say, a fresh Caesar salad with no dressing, there's very little (if anything) I can eat there.
And while I was just diagnosed fairly recently, it's something my coworkers/the party planners have known for weeks before I gave my notice, so it's not like we'd already made the plans and then I was diagnosed out of the blue.
Also, I would *love* to be able to pull off a "just this once" cheat, but that's really not possible on my new diet! All my doctors, not just my nutritionist, have sworn up and down that there's No Cheating Allowed.
So I think tomorrow I'll call the restaurant and ask about whether they could accommodate me, and if not, I'll just enjoy the company without ordering anything. I might bring my own food if the restaurant allows it, but I'm not sure I'm going to have time to cook anything extra between now and then!
One more question, though: If it does turn out I really can't safely eat anything there, would it actually be rude/ungrateful to just have a drink and leave early while everyone else is eating? (I hope I'll be having a wonderful enough time anyway that there'd be no need of this, but now I'm just curious.)