many thanks for your answers so far.
It is amazing how many really helpful and specific answers are posted in short time.
To summarize: through checkin FRA-LCY-JFK should be possible.
open question: FRA-LCY last evening flight, LCY-JFK next
morning, still possible when overnight stop
is involved ?

Regarding interlining AirBerlin to BA: VIE-FRA is a HG flight (FlyNiki)
Does FlyNiki even have interlining contracts with BA ?

(I remember once flying AA DFW-FRA and (on seperate ticket continuing) FRA-VIE with HG and AA didn't check luggage through because of "no contract with HG, only with AB (was a codeshare AB-flight number on HG metal)

Further answers / experiences are appreciated,
kind regards,

Ettrich