How do I run an interactive GUI application in pre-login environment?
I've been looking into this for awhile myself.  Mike Bombich been able to do it for years with his Bootpicker app.  And this developer is able to tap into running a GUI app as soon as the user clicks on a login icon: https://github.com/grahamgilbert/crypt (can't tell from his screenshot, but look up his OS X Management with Puppet presentation).