launchd daemons can use XPC activity just fine; XPC activity will start the daemon when it’s time to run the activity. The daemon will, however, have to run one in order to register the activity. The launchd property list keys for statically declaring activities are not documented for third-party developers, alas.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
Thanks Quinn. We'll do a RunAtLoad launchd.plist to install our activity for now.