How to modify an assembly before it is packaged in a XAP?

Question

I have a MSBuild task that modifies an assembly. At the moment I place the Task in the AfterBuild Target of the project file. Unfortunately this does not work for Silverlight because the XAP is packaged before the afterbuild is fired. The same problem exists for VSIX projects.

So how do I get this scenario to work?

Is there another Target I should be using?