After my recent religious conversion to WP, I've begun lobbying AirNZ for a UWP update to the existing WP mPass app.
It'd certainly be useful to be able to check in online from the app like I did with the Android version, but there's a whole host of additional features that would be useful across the app ecosystem.
Anyway, if anyone else wants to help, the email address suggested to me for contact was onlinebooking@airnz.co.nz or perhaps via Twitter.