Setting values before saving

We need to set a property exactly before saving. We use generic repositories. So, there is no chance to set it in the repository. The logical solution is events, but MongoDB driver does not have any “before save” event or any events that include the actual object, but BSON. I can’t find any extensibility point in the driver to inject any functionality. I tried sterilizers, decorating the collection…
I do appreciate any idea.