If you have a generic base class in a contracts library that doesn't have a dependency on Mongo.

public Thing<T>
{
  public T Value { get; set; }
  public Dictionary<string, object> Properties { get; set; }
}

Using the current BsonClassMap there does not appear to be a way to indicate that Properties should be used as the ExtraElements attribute without checking that the type has been mapped on each call and creating a mapping then.