Make something with a mod that dies shortly after it is spawned.
Add it to the GalaxyScenarioDef for your mod.
Make a copy of Galaxy Forge.
Copy your mods GalaxyScenarioDef over the GalaxyScenarioDef in the copy of Galaxy Forge.
Make a galaxy template in Galaxy Forge that spawns one of these death after spawn items for each player.
Add the template to every planet in your map.
Hope this helps.