It turns out you can trigger a 2FA verification 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 limited 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 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.

It turns out you can trigger a 2FA verification 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 limited 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 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 answer to thank them for helping out.

It turns out you can trigger a 2FA verification 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 limited 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 to get the verification code sent by SMS.

It turns out you can trigger a 2FA verification 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 limited 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 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.