> The issue isn't copying the artwork per se, it's copying the Vatican's picture of the artwork. They will likely release the images for free, but retain copyright over the digitisation.

In the US they can not claim copyright for this. This is very clear. See: https://en.wikipedia.org/wiki/Bridgeman_Art_Library_v._Corel.... and https://en.wikipedia.org/wiki/National_Portrait_Gallery_and_...