Do you know anything about programming in C++? If so you could probably do it in there (I am so new at it that this might be totally incorrect).
Lets say for the sake of THIS case (since I dont know the name of the function(s) used during character creation), that there is a function called Character_Creation. I would imagine all you would have to do is Set each different race that you didnt want used to enabled false. For example it MIGHT look something like this with dark elves:
Then, maybe write a new function that when a certain event is triggered:
<race in question> = TRUE;
Again, not TOO sure about that but it seems logical to my dumb ass <smile>. Still VERY early in the game for C++ and me. It may be WAAAY more complicated than that.
I dont know if you could just write a perl script to enable them or not, as Im not totally sure how THAT works either..