-I think you should be able to do this but you would have to deal with Microsoft support. (good luck with that)
-You can install the upgrade on a XP machine get validated then change your mind and transfer the license to another machine by calling Microsoft support
-I think you have to have a valid OS installed to get the upgrade price even if you wipe the machine during the install by deleting the partitions and making new ones. After the install is complete your license becomes tied to your machine signature during the Activation process.http://windows.microsoft.com/en-GB/windows-8/upgrade-product-key-only
-this link downloads the windows 8 image if you already have a key. It also indicates
that a valid OS must be installed (valid OS includes the windows 8 consumer Preview, or windows Developer preview)
-Some of retail full retail versions will not have this requirement but they cost more.
-my guess is that what you want to do will be painful and require talking with support to workaround the software limits put in to prevent clean installs with upgrade versions.