Code

<Style x:Key="boolCell" TargetType="{x:Type diag:TableNode+Cell}">
	<Setter Property="Template">
		<Setter.Value>
			<DataTemplate DataType="{x:Type diag:TableNode+Cell}">
				<CheckBox IsChecked="{Binding Tag}" Content="{Binding Text}" />
			</DataTemplate>
		</Setter.Value>
	</Setter>
</Style>

table[0, 0].Text = "check me";
table[0, 0].SetValue(TagProperty, true);
table[0, 0].Style = (System.Windows.Style)FindResource("boolCell");