Had this situation last week. Ticketed LGB-ORD-MIA. The best standby op was via DFW. I arrived at LGB 1 hour before flight time and was told that I could only standby for same routing. I pulled out trusty cell phone and spent 20 minutes with a nice res agent who siad he would get a definative answer (It was a web fare). The fare rules started "Standby allowed between SAME CITY PAIRS".

Went back to Ticket counter. Argued a bit, my early DFW flight had left, but they did get me on the next one.

Moral of the story- KNOW THE FARE RULES, because the AA agents don't.