Sorry jeans are not acceptable for non-rev on SQ
even in economy class. SQ
lays out very strict rules, as they do with everything, on the dress code.
To quote from their Manual:
Male: Shirt and pants. Coat and tie optional.
NOT ACCEPTABLE: Track suits, Jeans, Shorts, Tights, Singlets, T-Shirts, Sweat-Shirts, Slippers and Sandals.
Can't speak for our New Zealand staff but check-in here in Singapore with jeans and you will not travel.
Also sorry to say we are extremely tight fisted with our upgrades even with our own staff. Many times I have left behind countless SIA staff whilst departing with unoccupied First and Business Class seats.
Enjoy your trip.
Etihad Girl, You're a great way to fly.