As far as I know, Boot Camp doesn't support external drives (with a possible exception of some Thunderbolt devices and CD/DVD drives). The Mac's boot loader only supports booting via EFI on USB and Firewire, whereas Boot Camp is BIOS/MBR emulation.
Windows 7 x64 apparently has a UEFI mode. I don't know if this is compatible with the Mac implementation of EFI. It certainly requires a 64-bit EFI - so only Macs that are compatible with Mountain Lion are even candidates for this. In any case, I haven't managed to pull this off myself, and the information I've found on the web is conflicting. Even if you can get the Windows install DVD to boot in EFI mode (how?), Windows will probably refuse to install to a USB drive.
So in summary, there's certainly no straightforward way to do this. It might be possible with some effort, but I haven't seen any reliable reports of success. (and I haven't had the time to try it myself with Win7 - I tried and failed back in the Windows XP days)