I usually start considering these right about now, when we’re nearing release and things are pretty finalized. (like I did with the ender chest) So I haven’t thought about it too much, and you’re right. It’s dangerous. This is why I didn’t give out ghast, blaze, or creeper spawn eggs in Easter. Any destructive elements I restrict first and work in as I can. A regular restriction might be a decent way to go. Everyone else could still participate, but they’d require one person in the group to be a regular. Restricting right-clicking with wither skulls is easy enough. I could even just restrict it when placing it on soul sand, because you can’t do it any other way. That way people can still place them on other blocks, inside frames, or interact with chests/buttons while it’s in their hand.
I’m not sure if restricting where they can be placed is good or bad. Different terrains for the battle is part of the fun. Maybe we could restrict them to only be placed in outworlds where destruction is part of the fun. That way I wouldn’t have to create an additional distinction for regulars, which I hate to do.