After researching, I discovered there is no way to throttle a job on the fly anymore, but you can get a close equivalent by aborting and then updating the job with the throttle switch. Or, at least it should work in theory. It's not working in practice so far. I'm actually just manually running commands on my test data now at the cli and the throttle isn't 'taking'. I've got a case open with NetApp but would love to hear ideas anyone might have.
Commands I am running:
Throttle settings before change (while job is running)
snapmirror show -type DP -fields total-progress,throttle,current-throttle
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<source_path> <destination_path> unlimited unlimited 962MB
Abort job
snapmirror abort -destination-path <destination_path>
Update job with throttle
snapmirror update -destination-path <destination_path> -throttle 5120
Throttle settings show the throttle didn't 'take'
snapmirror show -type DP -fields total-progress,throttle,current-throttle
source-path destination-path throttle current-throttle total-progress
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<source_path> <destination_path> unlimited unlimited 864MB