Is your threat model really that people whose wedding you're attending are trying to exploit your phone..?

Conceptually I like the idea of giving everyone a locking pouch to put the phone in, with the ability to unlock it provided at the end of the event. That way the phone never leaves your person, but is unusable.

But at the same time this sort of thing would also make me kinda annoyed (as I have no problem, generally, keeping my phone in my pocket, on silent, when having it out is inappropriate0.

Not really, not if you secure your phone adequately, which these days is easier than ever.

It's unlikely there will be state actors at this wedding or whatever ready to open up your phone and swap out a chip.

Not really, a phone in first boot not unlocked state is secure against most threat models

Okay so leave it in your locked car...