It wouldn't be so much as treating Utena as 'chattel', as honoring her by she being worthy of bearing the mark of a god. They did specifically state she was all but becoming an 'adopted' Aesir (if more like dragged into becoming one). And since Gryphon used the AMG/OMG as one of the elements to make up the Asgardians, there's already a semi-precedence seen in the anime where a god/goddess' mark can be altered.