I called UA(SIN)in Nov/Dec about the possiblity of early check-in in Singapore. They told me that they only open 4-5ish am. In fact, I once arrived just before 5 am and the check-in counters were not ready.
If you do NOT clear customs you MIGHT have a shot at getting a boarding pass from the transit counter. If you are then lucky enough to get a business class boarding pass at that time, you can enter the Silver Kris lounge.
My suggestion, though, is to go have fun around town.
[This message has been edited by ddddd1 (edited 02-24-2002).]