I said this before and still think it. Don't go to the reunion at all. No part of it. Don't organize anything outside the normal stuff and especially something that excludes MIL (which you would have to do if you don't want her to wrangle her way after all).
It's only going to make you look bad. It's going to make you look vindictive and petty. "We shunned the event MIL planned and now we're rubbing it in her face." Plus, as others noted, there's the possibility of her finding out anyway and taking it as her cue to follow through with her original plan. Don't do it.
You have to teach MIL how to not cross your boundaries, right? Teaching this kind of lesson is hard and it takes sacrifice on your end in order to make her understand you mean what you say. In this case, the sacrifice is to completely forgo the reunion. I just don't think there's any other way.
When my kids were little, I left grocery stores, restaurants, libraries, etc. because I had to teach them that if they misbehaved we'd leave and that when I said that I meant it. It only took a few times but they learned. Believe me, neither they nor I wanted to leave these places. I really hated doing it. But it was a sacrifice today for behavior in the future.
And when I look back at all the subsequent pleasant experiences, it makes those sacrfices seem fairly insignificant. Of course, at the time, it's a huge hassle! But so worth it when you never have to deal with that particular behavior again, and when the child (or in this case MIL) learns that you mean what you say, in the future when you say "don't" she won't.