Try this (no guarantees):
<ComboBox >
<ComboBox.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
<TextBlock Text="{Binding SelectionBoxItem}"/>
</ToolTip>
</ComboBox.ToolTip>
</ComboBox>
To see when the tooltip opens, try something like this:
myCombo.Tooltip.Opened += new RoutedEventHandler(ToolTip_Open);
myCombo.Tooltip.Closed += new RoutedEventHandler(ToolTipClosed);