That's right!

All of the DA 7700 9 speed shifters (STI, bar end, down tube) can index with the 7402 8 speed derailleurs and cassettes. I feel this information isn't circulated much, as probably not many people try this, but it definitely works. I am currently running 7700 STI shifters with a full 7400 drive train on my Schwinn 754 (pics to come!) and it shifts great.