The July 2020 Mini Cooper Register magazine 'CooperWorld' has a feature with photos on restoring the now rare early 1275GT Chrome eyeball air vents. After a careful dismantling process the bright finish was restored by VMC a company in Chertsey, Surrey using a process called Vacuum Metalising or PVD (Physical Vapour Deposition), the parts were returned in a week and cost was £100 for the pair. I guess this process could also be used on the standard black eyeball vents.
https://www.vmclimited.co.uk/