Well, I've finally got the mod off the ground. As it turned out, editing the raceconfig file proved very necessary. Of course, for this to take effect, races must be cleared before they're used in a new game, or edited to make the range penalty take effect.
The next step from here is to look at nudging the priority of life-support technology. The mod I'm working on makes early life-support modules large (perhaps a little too large, I'm wondering) and all life support is a bit more expensive than it used to be. The more research you accomplish, the better things get. Miniaturisation will make a bit of difference to this situation, of course.
What concerns me is that this will may draw the game out unnecessarily, and as I've read the AI may not be able to adapt to this penalty and compensate by using starbases and colonising planets to extend range. I guess I'm only going to find out once I try a game