How about using some validator logic for this?

public class PageDataValidator : IValidate<PageData>
{
  public IEnumerable<ValidationError> Validate(PageData instance)
  {
    // TODO:   manipulate instace properties.
    return new ValidationError[0];
  }
}

I have used this approach to make sure the URLSegment property is always lower case. To validate other content types, simply change the type of class to validate in IValidate<T>

Probably I didnt describe clearly, what I'd like to do is to hook into Published event while publishing a block.

Ok, I sorted it out, there're also some equivalent events for block in DataFactory like: SavingContent, PublishingContent ... 

If you do not want to access the events through direct access to DataFactory you can aslo find them defined on the interface IContentEvent which is registered in IOC container

Very nice Johan!

Thanks Johan for this tips. I was looking for this interface ages ago, but no luck. EPiServer SDK still need to be improved :)

Small correction. The interface name is IContentEvents :)