When extending existing MSBuild targets, its often wise to append to the build target properties rather than override an existing target.  Its appears there's no way to override a target and then call the base target, this really annoys me because often I just want to add additional behavior to the existing target.  It seems the way around this is to add a property or itemgroup to the DependsOn attribute of a target, even if the itemgroup is empty, it just leaves a place open in the future for extensibility.  Here's in example from my shared targets file:

<Target Name="CreateDiskImage" DependsOnTargets="@(BeforeDiskImage);Build">

Now in some script which imports this targets file I can just declare one or more BeforeDiskImage items and they will automatically get run before the CreateDiskImage target.  If you look at the the MS providded targets files you will see this is done in a similar manner all over the place.  Here's a good reference for the C# targets you can hook into:

http://blogs.msdn.com/msbuild/archive/2006/02/10/528822.aspx