Yes you can. Provided you have no check-in luggage (though it's okay too even if you do - but it takes a bit more explanation).
First, see their site:
http://www.airport-hotel.com.sg/
Note that there are 2 Transit hotels. One in each terminal. NW arrives at Terminal 1. Instead of emailing them , CALL them. The email reply takes around a week!
The website is pretty detailed.... but yes, do not clear immigration and customs and follow signs or ask for directions upon arrival.
Slightly OT, but was considering using transit hotel for a late arrival with NW and early morning departure with Valuair. Apparently interline is not possible for check-in luggage and LCCs so am a bit concerned about leaving my luggage until morning. Could you explain how to claim my luggage in the morning (by notifying someone at the transit desk perhaps?)