It's annoying, but the best way to do it is something like:
<DataTemplate Name="MyCoolDataTemplate">
<Grid Loaded="HookLVIClicked" Tag="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListViewItem}}}">
<!-- your code here -->
</Grid>
</DataTemplate>
Then in the code:
public void HookLVIClicked(object sender, RoutedEventArgs e) {
var fe = (FrameworkElement)sender;
var lvi = (ListViewItem)fe.Tag;
lvi.MouseDoubleClick += MyMouseDoubleClickHandler;
}