DataMapper ORM v1.8.2

[eluser]mrflippy[/eluser]
I have a requirement where I need to not only set an update timestamp in the database, but also the username of the person doing the update (and possibly update some other fields as well). Obviously there's no built-in function for this like there is for the update date, but I was hoping to leverage the custom validation/prep functions. It looks like the validation functions are only run if the field value has changed, which, of course, never happens. Is there a way to update fields like this automatically, or am I stuck making an extra function call every time I call $model->save()? I guess I could always hack something into the core DataMapper object, but I'd rather not do that if possible.