It turns out you can trigger a 2FA [verification][1] code by going to:

iOS10: Settings > iCloud > Password and Security > Get Verification Code

iOS9: Settings > iCloud > [Your name at the top] > Password and Security > Get Verification Code

Now the challenge here is that the original iPad Retina [was][2] [limited][3] to iOS9. 

The other challenge is that the iPad retina running iOS 9 appears to have a 4-character 2FA - whereas my other devices are generating 6-character verification codes. I'll try and solve that somewhere else. 

There appears to be a [trick][4] to get the verification code sent by SMS.

---

Edit - it turns out it was asking me for the login passcode, not the iCloud verification passcode. Going to give the tick to the other other to thank them for helping out. 



  [1]: https://support.apple.com/en-au/HT204974
  [2]: https://discussions.apple.com/thread/7674401?start=0&tstart=0
  [3]: http://appleinsider.com/articles/16/06/13/apples-ios-10-restricted-to-devices-with-a6-soc-or-newer
  [4]: https://apple.stackexchange.com/a/267005/753