Hi Heiko,
Thank you for the provided details.
Your suggestion is meaningful, and this TagModel would be a good improvement for the RadTaskBoard control. Your request is approved. Until this functionality is created, you can implement this using custom code. You can check our TaskBoard -> First Look demo in our Telerik UI for WPF Controls Demo, which demonstrates how you can create a custom tag template.
Regards,
Dinko
Progress Telerik