It's actually some clever slight of hand with FRED and models and textures. There's an intermittent ship class with an animated diffuse map that lasts for 15 seconds at a very low framerate.
When the beast infection beam hits the heavy cruiser, a change-ship-class command changes the cruiser to the version with the animated map and damage lightning is added for good effect. After 15 seconds a second change-ship-class replaces the ship with the Beast Heavy Cruiser.
The problem is, half the time this crashes FSO randomly, so if this IS used, its only going to be on a select few missions and orchestrated in FRED. There probably won't be any random ships that fly around and get infected.