Well, you won't get to shoot the interiors. Below is the reply from the Istana office.
You can bring cameras into the grounds but not into the buildings
To: ISTANA General
Office/ISTANA/SINGOV@SINGOV.com
Subject: Regarding cameras
10/22/2003 03:05PM
Are there any restrictions on cameras during the open house this Deepavali?
Is the public allowed to bring cameras in?