Off the top of my head, there are some codeshares such as LAX-CMH (AA 1735/AS 6537) that are bookable without an AS segment.

Now that I've started down this road, it looks like there are a variety of AA LAX codeshare flights where stand-alone bookings through AS are possible:

ATL, AUS, BDL, CMH, DEN, ELP, IND, KOA, LIH, BNA, MSY, RDU, STL, YYZ, YVR, EUG, XNA, FAT, IAH, MCI, OKC, OMA, SAT, RNO, SMF, SAN, TUS, ASE

I just chose some random days in January 2018 for that pull. Out of LAX, it looks like any destination that AS/VX don't serve themselves with the noticeable absence of pre-merger US hubs is fair game.

LAX seems to be the most open hub to this sort of independent codesharing. I know AA ORD-BWI/DCA codeshares exist, but these look like they must be booked in conjunction with AS segments.