All right - I did some preliminary testing, and I think it'll work OK. I'm attaching a file with some of the appropriate changes; you'll have to cut & paste into the appropriate data files (and do a fair amount of modding to the race files, too; may even have to alter the "*.emp files" if you want to use the "Quick Start" option).
First, you add three new racial traits to "RacialTraits.txt", one for each Colonization Tech; obviously, give each one it's own racial trait ID. I set the cost to 0 (zero) so it wouldn't unbalance the existing setup options. Of course, it would be up to the player's discretion not to abuse this, since it would be fairly involved to allow the AI to start using all colonization techs again after following the rest of these steps...
Second, edit the colonization techs in the "TechArea" file to include the appropriate Racial Tech ID.
And here's where it gets tricky: you have to add the appropriate racial tech to each race. I've used the Abbidon in the example in the attached file; basically, edit the "*_AI_General" file so the race has one more "Advanced Trait" at each option level, and add the appropriate planet type as an advanced trait. For example, the original Abbidon_AI_General file reads:
=========
Race Opt 1 Num Advanced Traits := 1
Race Opt 1 Adv Trait 1 := Psychic
=========
And the new file would read:
=========
Race Opt 1 Num Advanced Traits := 2
Race Opt 1 Adv Trait 1 := Psychic
Race Opt 1 Adv Trait 2 := Gas Giant Race
=========
This is clearer in the attached file. I'm not certain what would happen in games where racial points are set to 0 (zero) for setup, but the above should catch the 2000, 3000 and 5000 racial point options.
Finally, all the "*_AI_Research" files need to be edited to remove all references to colonization techs.
You'll also have to pay attention in startup and select the appropriate racial tech for your environment; I wouldn't suggest selecting "Rock/Oxygen" as your environment, and then taking "Gas Giant Race" as a racial trait
.
Any comments on my plan for implementing Barnacle Bill's idea?