There is no way to trigger the db rule if the update is done at DB level. Actually the only thing that can be triggered when update at db level is a db trigger but I doubt that that's you want.
Instead of updating at DB level why don't you use data exchange? Updating through data exchange ensures that the db rules are triggered.
The procedure ought to be quite simple, the most difficult part is to establish a common "field" between OVSD and probably your HR software (or wherever the persons and there leave dates are stored) to use this as a unique key.
In your case I think that Data exchange should be easier than sd_event. Further more the data exchange guide has a through example of how to import data from an excel file.
My proposal is to create an excel with the persons and the leave dates (one per person) and use data exchange to import from excel.
Have a good read of data exchange guide, found on the doc\ folder on the installation CD. There you can find a through example of importing persons and their relations, your job is much easier since you haven't to import any relation. This guide is a pretty good starting point.