Have you thought about bringing this up on the systemd issue tracker? A target like that could be also standarized. Systemd also has an optional time sync service, that we already use by default, so this would be relevant to them and to us. At least we would get their feedback on the topic.
This should already works out-of-the-box with systemd-timesyncd: https://github.com/systemd/systemd/blob/master/src/timesync/timesyncd-manager.c#L619