Actually, Diego Garcia (the British/American military base) already has it's own ISO 3166-1 alpha-2 code, DG: [1]

It's "Exceptionally reserved" "at the request of International Telecommunication Union (ITU)".

Although my gut feeling is that if the deal happens, Mauritius would probably ask to exceptionally reserve `IO` as well. Still, I'm not sure if I'd risk using a `.io` domain since it's not guaranteed.

[1]: https://www.iso.org/obp/ui/#iso:code:3166:DG