With FXML it is possible to add custom properties to any component such as:
<Button ...>
   <properties foo="123" .../>
</Button>

that feeds the observable map of properties of the component. It works fine when doing it by hand, but when the fxml file is edited with Scene Builder, the properties tag is just removed. The first improvement would to do not remove it, and the second would be to allow its edition within Scene Builder.