Currently, the placeholder cannot be changed to anything custom. It is defined automatically based on the format value. It cannot be hidden too. It will be displayed if the component doesn't have value or it is partially entered.
A possible workaround is to set the value of the input element to empty string before the serialization and then restore the placeholder:
Another solution is to modify the scripts locally to not set any value in the input. The _updateElementValue
private method of the DateInput (kendo.dateinput.js) widget sets the value.
Have in mind that we do not recommend modifying the scripts as only the current state is tested, also any local modifications will be lost after an update.