There have been several attempts to make BartPE or UBD run form a pen drive. none as far as I know have been entirely sucessful, but I havnt kept up with such activities for a while so I am not sure the status of such efforts.
There are good reasons why most dont bother with such an undertaking.
1. All motherboards do not support booting from USB. but they do support booting from CD.
2. plugging a fragile usb flash drive into the usb port of a possibly damged machine falls under the category of "stupid things to do, if you want to trash you flash drive."