How can I have a multi-bootable USB KEY that has installers for many different OS's?
This article :
Boot Multiple ISO from USB (MultiBoot USB)
describes doing so using MultiBootISOs.
Grub4Dos is a great way to make a multiple boot external drive. I just finished designing the menu for mine the other night. You can change the colors up and even insert a low res splash image into the background. Pretty simple to use. Plus its FREE
As far as your question about embedding a product key. I only know about daz winloader that automatically activates any copy of windows although it doesn't work on windows 8 and Im still looking for an answer to that one.