I have done standby on earlier flights with checked baggage and I do not think I was ever told no. Every time I was told that my baggage will be loaded onto earlier flight regardless of if I will clear standby or not.

I did standby on earlier flight once on US before official operation merger with AA. At first an agent told me cannot do that at US, but the agent talked to a supervisor (the name tag said so) and did few stuff on the computer and let me stand by on earlier flight with a baggage. Looked like the agent did not know how to do it rather than not possible at US. At US they put some tag on the baggage tad, the agent explained to me that the baggage handler would bring my bag to the aircraft but would not load the bag unless they receive a call from a gate agent via radio before the departure. The agent told me that was the way it worked at US.

Yes, when I standby on earlier flight with baggage, I would have liked to be at the destination earlier to get some work or other thing taken care but did not need content of my baggage immediately upon arrival at the destination.