How do I get an existing installation of Windows 7 working from a USB 3 external drive?

I'm attempting to copy my existing installation of Windows 7 from my work laptop to an SSD of my own, and boot from it from an external enclosure via USB 3.0. I've used the laptop's internal SATA ports to do the actual migration, and the new drive works fine when used as the sole internal drive. But when I try to put it into the enclosure and boot from there, I get a bluescreen during the boot animation (0x0000007b).

I believe the problem comes down to Windows not knowing about USB 3 before it's fully loaded. Evidence: I can boot from an install medium, and the enclosure isn't visible till I explicitly load my laptop's USB 3 drivers.

So: How can I get those drivers to load as part of Windows' boot sequence early enough for this to work?

It's not a problem with lack of USB 3.0 drivers. With the exception of Win8 Enterprise's Windows To Go feature, Windows does not allow installation to or booting from USB drives without jumping through hoops. Unsupported methods using WAIK, ImageX, Sysprep etc. do exist for fresh installs, but I don't know of any way to get an existing Win7 install to boot from USB.