Is there a built-in way to lock a post / reply so that it can only be edited by its non-moderator creator, let's say, within the half hour after it's first posted?

You cant do this by content, but you can do this by group. So if you edit any group, you will see on the Content tab, under the Editing section,  there is an "Edit time restriction" setting, which I believe is what you are looking for

Fantastic, thanks Marc 🙂

4 minutes ago, Vero Pepperrell said:

Fantastic, thanks Marc 🙂

No problem at all. Glad you have what you need 🙂