Yes, it is possible to trigger a SnapMirror update from an external system. You can schedule it using whatever you choose...e.g. cron. There is a number of ways to trigger the update operation:
SSH with key based authentication to simply issue the "snapmirror update" command for the target volume.
For Linux, you can use a Perl/Python/Ruby script with the NMSDK. There are examples in the documentation of how to get SnapMirror status which require only simple modification to do updates.
Whether or not it's recommended is a different matter...there's nothing that says you can't do it, or that you shouldn't do it. It's completely a matter of preference and your own policies on whether you want an external entity starting the job. One thing to consider...make sure you document somewhere well known which host is starting the update job and how it's configured. There's nothing more frustrating than having a mystery host starting jobs which could affect other things in the future.
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