You know, having trawled through a bunch of mod code and not having ran into any math.Random calls made me assume the determinism requirements prohibited random values BUT IT TURNS OUT I WAS A FOOL.

Thank you very much!

Edit: Ah, and now I see the line about it being reimplemented for determinism ...

The noise in velocity and position of the resulting downsized asteroids when asteroids are destroyed appear to be accomplished by the offset and offset_deviation keys inside the dying_trigger_effects of the entity-type asteroids (used on both the create-entity and create-asteroid-chunk type effects ...