Actually, I'll be honest, I'm no longer getting a method-not-found exception but my scripts registered this way aren't actually getting executed at all, whether isolated or in MAIN. Are there any sample projects that are known to work available anywhere? If I put the script I am trying to execute in the manifest, it executes fine. I have, of course, remembered to put the script in web_accessible_resources but I'm getting neither execution nor errors.

Of course it does... Turns out I had some strange cruft after trying to get things working. My real problem was coming from Safari not using InjectionResults when returning from browser.scripting.executeScript and just returning the results directly. After fixing that, it turns out the registerContentScripts was working... Sorry for the noise! However, someone should update the MDN because, as you point out, Safari 16.4+ RegisterContentScript does support the world parameter!