Seems it is impossible to use RadTabItem.HeaderTemplate if RatTabControl is not binded to collection.
Using Snoop i see that template itself is applied but DataContext is null. So any binding don't work in the template.
http://docs.telerik.com/devtools/wpf/controls/radtabcontrol/templating/item-templates-and-selectors say "The HeaderTemplate is mainly used when we bind the RadTabControl to a collection". ). It doesn't say that it is used ONLY for collection.
HeaderForeground seem is ignored completely. Using Snoop i see that foreground is taken from some resources if i don't use HeaderTemplate.