You should consider using a custom action or installscript.
Since I write the ugliest InstallScript around, I can't help you there. I can suggest running a VBScript. There are plenty of free scripts out on Microsoft.com or Winscripter.com that deal with streaming text into a file.
http://www.winscripter.com/wsh/fileio/ht_createfile.aspThe InstallShield help files can help you get the custom action running.