The real problem with that is that the explosionDef.entity only supports hardpointed particle effects...
I have been playing around with that and I think that creating a shatter effect would be possible, but it would require massive amounts of work/pre-shattered parts that are arranged in such a way that it looks nice, not to mention the particles themselves.
What would be cool is if the large peices started floating towards the planet, real slow. When it crashes, it does some damage to the planet. The pieces could even be a target for your ships. They would kill the living ships first, and then turn their guns on the pieces.
this would be cool, but maybe a wee bit too complicated?