Currently the dragged item from the palette is not customizable. Therefore dragging an item from palette on MacOSX will automatically be displayed as a rectangle of size of the boundary of the dragged item in the Palette. There are some specialized scenarios where you want to disable or change that default rectangle e.g. when you are dragging an item from Palette and dropping it into a visual-designer e.g. Java Matisse or JavaFX Composer. These Design areas are rendering the dragged item directly in their area and with better details (e.g. an actual component instead of a Rectangle).
Therefore there should be an API/SPI which allows to customize it by the owner of the content of the Palette.