How to add custom-control-derived TabItem to TabControl in WPF?

Answer 1

Ok, I'm stupid, it should've been

<MyNs:MyCustomTab x:Class="MyNs.ActualTab"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:MyNs="clr-namespace:MyNs">
<Grid>

</Grid>
</MyNs:MyCustomTab>