The options in <field> attributes are parsed as python expressions: d18976d748/addons/web/static/src/js/services/data_manager.js (L273) And the options in <button> are parsed as json... d18976d748/addons/web/static/src/js/services/data_manager.js (L411) This code only support the <field> element because I'm not sure there is a use for the <button> element.
d18976d748/addons/web/static/src/js/services/data_manager.js (L273)
d18976d748/addons/web/static/src/js/services/data_manager.js (L411)