Registration format
Each country has a format for registering planes there - for example in Canada it’s C-four letters, while in the US it’s N000IATA code. So this could require airlines to give a two digit IATA code for the US at least, but basically how it would work is when airlines go to reregister an aircraft, they will only be able to register it in their base country’s format. Might be more difficult in the EU - separate AoCs would have to be implemented first for the EU to work properly.