Yes, that works. Thanks! I was missing how to set the "BasedOn" property correctly:
BasedOn="{StaticResource {x:Type ComboBox}}"
So adding this ComboBoxStyle with the "BasedOn" property above as a static resource and setting all the ComboBox styles to it is giving me the exact behavior and look that I was expecting:
<controls:ProWindow.Resources>
<ResourceDictionary>
<Style x:Key="ComboBoxStyle" TargetType="{x:Type ComboBox}" BasedOn="{StaticResource {x:Type ComboBox}}">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=DescriptionCheckBox, Path=IsChecked}" Value="True">
<Setter Property="DisplayMemberPath" Value="Description" />
</DataTrigger>
<DataTrigger Binding="{Binding ElementName=DescriptionCheckBox, Path=IsChecked}" Value="False">
<Setter Property="DisplayMemberPath" Value="ManiceCode" />
</DataTrigger>
</Style.Triggers>
</Style>
<ResourceDictionary.MergedDictionaries>
<extensions:DesignOnlyResourceDictionary Source="pack://application:,,,/ArcGIS.Desktop.Framework;component\Themes\Default.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</controls:ProWindow.Resources>
<ComboBox x:Name="cboCT" Style="{StaticResource ComboBoxStyle}" Width="60" MaxDropDownHeight="540" HorizontalContentAlignment="Center" ItemsSource="{Binding Concentrations}" SelectedValuePath="ManiceCode" SelectedValue="{Binding CT, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />