This might not be an ideal solution, but you could define the trigger in your control template. Don't forget to reference your TextBlock with the TargetName
property on the setter.
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<TextBlock x:Name="TextBlock">
<ContentPresenter Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
</TextBlock>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="true">
<Setter TargetName="TextBlock" Property="TextBlock.TextDecorations" Value="Underline"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>