How to clean install Windows 8 Pro *Upgrade* on BootCamp already running Windows 7?
It should be possible to do the clean install of a Windows 8 Pro upgrade on an existing installation of an earlier qualifying OS as these steps for Windows 7 suggest:

http://www.tuaw.com/2009/11/02/an-illustrated-guide-to-boot-camp-and-windows-7/

In this guide above, search for string: "Theoretically, you're doing a fresh installation of Windows 7. In that case, select the "Custom (advanced)" installation" 

(remove just the outermost quotes when searching for the string in the guide.)

In summary, after the Mac BootCamp app has done its original setup (including the partitioning), the rest of the setup is as per usual Windows installation - all the Windows steps are shown. In the event of the usual ReBoots, be sure to select the Windows drive to continue the installation.

During the install, once the Windows 8 upgrade installer has found the previous qualifying OS on the disc, it can then offer the format disc as the clean installation option.