How to install Windows 8 with BootCamp 5 Assistant via USB?
This doesn't exactly answer your question but may help solve your problem: create a bootable Windows DVD from the Windows 8 setup files, as explained  here.

I did it myself some months ago when upgrading Windows XP to Windows 8, and it worked without a hitch.

You will need:


Access to a Windows computer.
oscdimg.exe, a command-line tool for creating bootable Windows ISO files. This file is part of a larger download:


For Windows XP SP2 with KB926044/2003 SP1 with KB926044/2003 SP2 you'll need the Automated Installation Kit at http://www.microsoft.com/en-us/download/details.aspx?id=10333
For Windows 8/7/2012/2008 R2/Vista/2008 you'll need the Assessment and Deployment Kit at http://www.microsoft.com/en-us/download/details.aspx?id=30652

Windows 8 setup files.


The steps are:


Download and install oscdimg.exe.
Put the Windows 8 setup files in a folder. While any name will do I found C:\WIN8AIO to be a popular choice, so I'll use it.
Open a command prompt as administrator and create the ISO image C:\WIN8DVD.ISO like this:

\path\to\oscdimg.exe -lWIN8DVD -t10/26/2012:00:00 -m -u2 -bC:\WIN8AIO\boot\etfsboot.com C:\WIN8AIO C:\WIN8DVD.ISO


For more information on oscdimg.exe and its command line options see this article at Microsoft.
Burn C:\WIN8DVD.ISO and use it with Boot Camp Assistant on your Mac.