Sure you can. On the AS website, you can book AA routes like LAX-ATL, LAX-AUS or PHX-GEG as standalone flights. Not sure why they don't sell all the domestic codeshares--there are even some I've seen that you can't book. SEA-LGB sends you through Phoenix, but only on one flight. AS codeshares on all three AA PHX-LGB flights.

But hey, I can book LAX-RNO-SEA-DFW-CLT for $2,800 in coach (and an extra $75 for an assigned seat on the three AA legs).