Hey ronin I have 2012 GU8 too. I have had one service with Nissan which I took the plate out for just in case it through the engine light. I don't think the codes are stored, once it's cleared it's gone. If you service your car at a Nissan dealer you will be lucky if they actually change your oil let alone go searching for cleared codes in your ecu.

Also as far as I know they have to prove the modification caused the issues if they don't want to honour warranty.

One final thing your warranty is still valid as long as your car is serviced by a qualified mechanic. So of you don't want to pay ridiculous prices and want your car serviced properly take it to a good mechanic you trust and only takeout to Nissan for warranty issues. Once you have put in the egr block it will only take you twenty minutes to pull out, So it's easy to remove