Sorry that I didn't know about SQ, it wasn't in the list in the quoted email, and I never use upgrades on SQ, instead I buy the ticket that I want in the class that I want to fly in.

Asiamiles does allow upgrades on more than 1 OW airline, and several years ago AA offered mileage upgrades on CX. So there is past precedent for offering cross-airline upgrades within OW, but also precedent for removing these awards. My sense is that OW member airlines compete with each other a fair bit (sometimes more than with Star and Skyteam) and use upgrades as a competitive tool. Which makes it less likely for a OW system-wide upgrade to ever happen.