"I just want to point out that the official Geass-net mobile site explains in Nunnally's profile that her blindness gives her the ability to see through the hearts of people just by touching their hands, and in the final episode she found out everything with said ability. Officially, that flashback scene had nothing to do with Lelouch and Codes.
C.C.'s profile has also been updated, and the final part of it now says: "Knowing that Lelouch does not hate her for giving him the Geass, she is now able to show her true feelings. With the realization of "Zero Requiem", her time with Lelouch, who was able to forgive and accept her, came to an end, but the memories created with him has, without doubt, saved her from eternal loneliness."
Having read this, C.C. appears to have been talking about herself when she said this line in the end: "The power of the king will isolate you... I guess it turned out a little different after all... right, Lelouch?". The power of the king eventually leads to immortality (Code); C.C. had the power of the king and for a long time led a lonely existence, but she is now no longer lonely, thanks to her time with Lelouch."
Unless they update Lelouch's profile with some kind of "but maybe he didn't actually die" line, it seems to me that officially, Lelouch is 100% dead. None of the magazines seem to think the ending is at all ambiguous, and none of the staff and cast members seem to think so either. Okouchi, especially, appears to be pretty final about Lelouch's death in his interview ("His life had not been in vain. That was why he smiled in the end", along with his explanation as to why the punishment was necessary from the beginning). As for Taniguchi, he said only 2 things: (in the NewType interview itself) It's up to the viewers whether they want to think of the ending as a Happy End or a Bad End, but he thinks of it as a Happy End. As for what the epilogue meant, the viewers ought to work it out for themselves. The meaning behind C.C.'s final line appears to be part of what he wanted viewers to think about, though Geass-net's already given us the answer to that. Lelouch's fate, is, IMO, not part of what he wanted viewers to think about, because as far as the staff members are concerned Lelouch's fate wasn't at all ambiguous.
By the way, Shimomura Takaharu, one of the staff members involved with advertising, wrote in his "Parting message to Lelouch" (from Animedia):
"Let's all go to the World of C" (For more details, refer to the R2 drama CDs)
Looks like SE6 will feature a track about everyone going to the World of C.
Tomikawa Kiyoharu (Production Desk) has this to say to Lelouch:
"Rest in peace. I'm begging you, please don't come back ever again. (The animating process was just too much trouble!)"
Most of the staff members' parting messages for Lelouch are pretty hilarious, and are in keeping with the Geass sense of humour.