Josh:
It looks like only root can modify scripts in the PreSession directory, but I don't have root on this system. Does GDM support a user script like this that I can set up without root access?
Correct. You need root access to modify these files. You might be able to use your $HOME/.profile or $HOME/.cshrc file (depending on which shell you are using) to set values you want to set when you are logging in. I think these are the only files that users without root access can modify to affect their login. Brian