It might be tough for a principal to do it specifically. We've got a group of teachers specifically given the position of "culture movers" in the school and I work with them to lead groups in which they establish these norms. I think figuring out who your connectors/influencers are is the first step--if you can get them to be leaders in small group work, I think that will bear more fruit.
The easiest part about it is that it's rooted in perceived teacher expertise. Teachers are always interested in helping each other, especially when they feel like the topic is in their wheelhouse. In reality, I'm not 100% sure how well we actually helped with any of the individual challenges, but the process brought people together.
Probably #1 was keeping the time to 15 minutes. As people got more and more into it, a lot of the ideas starting rolling through into 20, then 25 minutes. I had to get better at stopping the conversation, unfortunately.
The other piece was the element of "brainstorming" that the slam entails. The idea that we weren't going to solve this issue in the time allotted, nor that we were really looking for perfect solutions, so to speak, was difficult for some to grasp. It took awhile for people to get comfortable just riffing on ideas, asking following up questions, and generally just helping the person who proposed the challenge get a different perspective on it.