Meph, I imagine you're aware of the 40.04 tree problems, where they drop limbs capable of soil penetration and shoot up through soil and rock. You might want to hang onto a copy of 40.04 if you think you can duplicate this bug as a feature on certain creatures or a new megabeast. Because a bunker buster from either above or below on anything makes it a vicious late game opponent if you wanted to up the difficulty.
Edit: Or, if it can't be applied to the creatures directly, you might be able to make them spawn the bugged version of the trees. These suckers grow in about a week's in game time and are perfect for an invader to rip open defenses and force their way inside.