Thanks for everyone's replies.
If I understand correctly, as long as I cancel the "connecting" hkg-nrt segment (which would be illegal due to stay longer than 24 hours), I won't have any problem using the rest of the ticket?

I guess the airline won't track whether I used a different carrier or routing.

Of course, if I end up in that situation, I'll probably just try the walk-up scenario. It's worked for me many times with out of order coupons and expired (more than 1 year old) tickets. I've just never tried it with a rtw and didn't want to get socked on one of my later flights with a some huge unexpected charge. With an old coupon, I know my worst case is simply being charged for a new ticket. I wasn't sure about the worst case on a rtw.