You can try, but the policy is no.
Good luck.
I am in LHR, just got in from LAX. And flew AA.
I am a furloughed AA employee. And even for active employees, if you want to upgrade you have to pay for it. But not for free.
But if you are a crewmwmber for a US carrier and you are jumpseating you could get a premium seat if the purser and the Captain offer it to you. otherwise the policy is no.
Later