I agree with the sentiment regarding OOTB functionality (UpdatePolicy) being preferred to a rule.
However, what I have found very useful in the meantime is to use the Services Standard Before Provisioning Rule. The rule can be re-used on sources of different types and since the “UpdateAttribute” function is configured in the source’s connector attributes, the additional attributes can be updated easily at any time without changes to the rule’s code which would need re-deployment.
The only drawback I have found is that this method generates a “Modify Account” event for each additional attribute added, which can be a tad confusing in logs and audit when there may have only been one sync attribute that was truly modified. If anyone has found a way to work around this, I’d be curious to hear.