Haswell 15" MBP, Win7 x64, no trackpad or keyboard
After playing with this for a 8-10 hours, I found a solution:

You have to have Bootcamp Assistant create the USB key, and download the latest drivers (first and second checkboxes) to the key. There's apparently some startup information created that will let the keyboard and trackpad work with the installer.

Once that's done, boot into the Win7 installer. It still probably won't install. However, at some point you'll be presented with an option for "Load drivers," which appears in a few different places during the install.

When you get the opportunity,


Browse (on your installation media) to $WinPEDDriver$\IntelxHCISetup\Drivers\xHCI\Win7\x64\
Uncheck the box that says "hide drivers for unsupported hardware." (Yes, really.)
Select everything it finds.
Hit next. Wait for the drivers to load.
Exit out of the installer, closing everything until you get back to the "Install or Repair" screen (first thing you typically see when the Win7 installer boots. DO NOT REBOOT
Remove your USB key.
Wait for a moment.
Re-attach the USB key.


Proceed through the "Install Windows" wizard, select your target partition (probably have to reformat it as NTFS, depending on how you created it), and proceed as normal.

There appears to be an issue with the way that UEFI handles USB 3, so this may not be an issue with a USB 2 key.