Unfortunately this is not possible in the current OCUM 5.x. But its possible to write a simple script plugin to report and monitor on the same.
BTW why do you want to monitor the same ? What do you plan to do based on this event ? I am trying to understand why and what you are trying to accomplish. Given the fact that aggr scrubbing is an internal ONTAP process.
This isn't an exact way to determine when it finished, but using Performance Advisor is a great way to visually see when a raid scrub started and finished. By examining the aggregate (under physical objects for a given controller) and looking at the "Aggregate Summary View", set the time range so that the appropriate day is shown on the chart. You can always tell when a RAID scrub is occuring by the large spike in aggregate transfers that sustain for the entire length of time the RAID scrubs runs. Its followed by a sudden plunge in aggregate transfers once the RAID scrub stops.
Unfortunately, it would be very difficult to create a performance threshold that tells you when the RAID scrub starts & finishes, as Performance Advisor has no way of knowing what is causing the aggregate transfers. It just tracks the number of transfers.