I like it. They could request the judge that he serves his time inside the museum itself, in a cell prepared just for him where visitors can observe his daily routine. He will not only participate in one of the greatest modern art performances of all time, but he will indeed become the artwork.
It's almost like you're saying that an NFT doesn't actually capture the thing it's correlated with. But if that was true it would completely undermine the concept of an NFT of an art work. The NFT wouldn't have any more unique value than an item # in a catalog.