I am flight LHR to SFO via ORD (B class fare) this Thursday at 3:40pm

I was not able to book either of the non-stop flights, as they were sold out in coach.

Is it permissable to standby for one of the non-stops?

They are showing the following availability:

UA 955 at 10:05am
F8 C9 D9 Y0 B0 M0 H0 Q0 V0 W0 S0 T0 K0 L0 G0 P8 A7 Z8 E0 U0

UA 931 at 2:15pm
F7 C8 D4 Y0 B0 M0 H0 Q0 V0 W0 S0 T0 K0 L0 G0 P7 A6 Z3 E0 U0