Hi Volker,
Greetings from Syncfusion support.
We can achieve the requested requirement by using RenderDayCell event in the DateRangePicker. This support will be included after hybrid support implementation for DateRangePicker which is included in our Volume 3 Main release scheduled to rolled out on end of September 2020. We appreciate your patience until then.
You can track the status from the below feedback link.
Regards,
Berly B.C