Thank you for your interest in UI for NativeScript.
At this point, there is no functionality, which allows setting placeholder animations. Regarding that, it would help if you could provide some more info about the way you are expecting a similar feature for a placeholder animations to work.
Also, I would suggest logging this feature in the nativescript-ui-feedback
repository, where you could also describe the way you are expecting this functionality to work.
Hope this helps.