That 'dress' is Brunei traditional costume or called 'Pakaian Brunei'. Yes, it is the same costume any Brunei bride (including Princess Sarah) wears during the 'Berbedak' ceremony. In this particular case, the ceremony is called 'Menghantar Tanda DiRaja'. I guess it's like the sending of royal dowry. The girl, fully attired, is one of items in the dowry (I guess a very long time ago, the girl would be the present of the groom to the royal bride, where she will stay in the palace with the princess) But now in the modern time, she is just a symbol to fulfil the royal tradition.
Btw, please refer to the website below for details of the royal wedding including the explanations of the ceremonies:
Royal Wedding