

I will test booting from rEFIt and update this answer when I have. I know for a fact that Unetbootin works for creating bootable Windows install USB's (I've run them before on PC's). (boot to a rEFIt DVD by holding down the C key after you hear the chime of the startup) Put that on a script or embedded on a PowerShell call and use the task scheduler to run it at logon/boot. PowerShell to the rescue: Mount-DiskImage. Having had trouble trying to install it to the Mac itself (without the DVD), I usually boot to the rEFIt DVD, and continue to boot to my choice of operating system. Otherwise, you can use it to create a virtual CD drive thats pointed to an ISO, and tell it to stay persistent on reboot. I recommend simply burning rEFIt to a CD/DVD to boot to the Windows 7 install USB. It is useful for recognizing operating systems installed on your computer and devices connected to your computer (and their partitions). I use it myself to boot from an Ubuntu partition on my external hard drive that I use as a Time Machine on another partition. To boot from your USB drive, I recommend rEFIt. Click the "." button to browse for your image. (note that Unetbootin requires admin privileges) The following image demonstrates how to manually select the ISO image.

Unetbootin allows you to create bootable USB drives from many Linux distributions and also ISO images. Assuming that you have the ISO handy (if you don't, there's Digital River links here), download Unetbootin.
