It's not really clear in the UA rules. It doesn't specifically say you can't, though:

NON-REFUNDABLE FARES
--------------------
FOR ALL TICKETS ISSUED NO MATTER DATE OF ISSUE OR TRAVEL:
-PASSENGERS HOLDING CONFIRMED RESERVATION AND TICKET MAY
STANDBY FOR ANY EARLIER/LATER SAME-DAY FLIGHTS FOR NO
ADDITIONAL CHARGE
-THIS INCLUDES AT MULTICITY AIRPORTS LISTED AS FOLLOWS
UNLESS RESTRICTED BY THE AIRPORT PROVISIONS OF THE FARE:
CHI CGX/MDW/ORD
NYC JFK/LGA
WAS DCA/IAD
-SAME-DAY STANDBY FOR ANY FLIGHT IS PERMITTED..INCLUDING
FLIGHT-SPECIFIC FARES AND CONNECTIONS STANDING BY FOR
NON-STOP FLIGHT ..AS LONG AS ORIGIN/DESTINATION AIRPORTS
REMAIN THE SAME.
-CUSTOMERS MAY BE PUT ON THE AIRPORT STANDBY LISTING
ONLY WITHIN FOUR HOURS OF THEIR DESIRED FLIGHT TIME.