Right now each faction has an entry along the lines of <TechTree>Terran_Tree</TechTree> in FactionDefs.xml. Between the StartingShips, CreditsInit, & race traits, you can configure/mod a lot about each faction, but you cannot set custom colonization events. Specifically I would like to see a <ColonizationEventsFile>somefile</ & maybe <IncludeDefaultEvents>true/false</>. If we are getting really out there, maybe the same options for custom IdeologicalGalacticEventDefs in addition/instead. Speculating on the inclusion that since the xml's are pretty much the same format, it might be possible to almost copypasta both if one gets done 
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