I received an OpUp last August on a Transcon from JFK to LAX (J to F) as a (non-GGL / non-AA status) vanilla BA Gold, so it's not impossible. Interestingly, I was already sat in J post-boarding when the ground agent came on board and asked me to take a seat in a half-empty F cabin as they "needed my seat". I remember being super surprised at the time that they would choose to upgrade a oneworld Emerald over one of their own elites but had no chance to ask "why me", as they were rushing to close the door.
My general experience on AA (in over 100 flights) has been that non-AA elites do not qualify for the official "upgrade list" (i.e. the list that gives "automatic" as opposed to "operational" upgrades to their own elites) and therefore have zero effective chance for an operational upgrade i.e. because the former list would have to be at zero for non-AA elites to be considered. In almost every domestic flight I have taken the "automatic" list always seems to be in double digits. I would venture the 3 class Transcons might be the exception i.e. the sole domestic flights where EXPs don't have an automatic right to a bump from J to F, as opposed to burning their limited guaranteed upgrade certificates, which from scanning the AA forum people seem to prefer to retain for international travel. I would imagine that I lucked out on a once in a blue moon day on which operational upgrades were necessary, there were no Concierge Keys or EXPs travelling and / or they wanted to showcase F to non-AA elite?? Who knows but I have zero expectation of it ever happening again. Between AA elites, friends & family, dead heading pilots / crew, etc, I would imagine oneworld elites have as much chance of an OpUp on a typical AA flight as winning the lottery. But I of course defer to those with superior knowledge...