FWIW just did a short haul on BA connecting to LHR-JFK-SFO on one rez, with BA flight being vanilla BA, not codeshare, and the others being AA. Checked in on BA.com yesterday for my CE short haul, could not check in on AA this morning as it wanted to redirect me to ba.com, but immediately after I landed at LHR today I was able to check on AA.com for my flights. It helps I have to overnight here so I have the option of doing this, which normally would not be the case if the flights all left the same day.

Says I have TSAPre for my LHR-JFK flight, bet ya that's not happening