You should be able to but at SIN I don't know why you would want to. The CX lounge is at the opposite end of the terminal from QF's (which is nearer to the gate you will probably be boarding from). From a quality perspective the CX lounge is nothing special.
Practical considerations aside, the January 2002 "Your guide to the oneworld alliance" published by oneworld states in the section on lounges:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Naturally, if you are flying Concorde, International First or Business Class, you always have access to the equivalent class of lounge regardless of your frequent flyer status.</font>
It doesn't specifically say you are limited to lounge of the airline you are flying.
[This message has been edited by mgm (edited 05-11-2002).]