Ok, here's the answer... after a couple of E-mails & a long phone conversation w/ my cousin who works in Redmond, for MS, who was, actually on the Windows7 Build Team & is, obviously close, in his position, to what's what w/ Windows8. Furthermore, before we talked, on my behalf, he took the query to his colleagues responsible for Win8 Deployment & Start-up. That said, NO, cannot use Upgrade to do a clean install on a blank HDD or SSD, for that matter.
However, you can take a blank drive, throw say, XP, Vista, Win7 or even Win8 RP on it and then install the Upgrade telling it to keep nothing which, will format the drive as part of the install process... this would be the recommended approach as it, in effect, amounts to a clean install. Keep in mind, if using the RP, you better already have the RP because once Win8 goes RTM the RP will no longer be available for DL.
The (old) "double pass trick" used w/ Win7 Upgrade discs will not be doable w/ Win8 Upgrade discs or .ISOs. W/ 8 a Key must be entered for the install to proceed & the delayed Activation question/option does not exist @ the beginning of the Win8 install, either. As well, remember you don't buy the software, you buy a license. All the SKUs are on an install disc (or ISO) but, it's the Key, the License that determines what you get. Ergo, if, you tell, assuming there the choice/option, it to do a "custom" install but, have bought an Upgrade License (Key), you will get told the Key does not match what you are trying to do. Plus, if, it doesn't 'see' anything, an OS, already on the drive, the Upgrade (install) will not proceed.
