The correct answer is: "it depends". Each fare rule has booking rules for interline segments, and if EF doesn't allow you to look them up then it's definitely something that they should offer (the info is in Sabre). In my own experience fares interlined with BA, are almost always booked in the same exact class that they are on AA, however izzik seems to have found an exception--maybe from many months ago.

The fare in question, S00A3M CHIMAA, does indeed book in S on BA. On DL, it would book differently depending on the market; on PARMAA it would have been either U or T (or, on the AF codeshare, N).