Similar to Counter-Strike, Left 4 Dead 2 allows you to write scripts which modify the game’s behaviour.
Unfortunately, there is no default file to include your modifications in; however, there is a launch option you can use to run a script when the game is loaded.
Add the following to your launch options [cci_cstrike]+exec scriptname.cfg[/cci_cstrike] to execute a script which should be placed in your [cci_cstrike]cfg[/cci_cstrike] folder.
As a side note, you may want to add [cci_cstrike]-novid[/cci_cstrike] to skip the movie when the game loads.
skeet. skeet.