I just completed a clean installation of Windows 8.1 using a Windows 8.1 with Update 1 .iso and a generic product key. 2 additional screens were added after I installed by signing into my Microsoft Account.
The screen to reserve:
After selecting reserve I was informed my free upgrade is reserved:
As you can see a generic key was used and Windows is not activated:
This suggests the use of a Windows 8.1 .iso with a generic key can allow one to get a free upgrade to Windows 10.
Edit: This is only a marketing stunt and the free upgrade requires an activated base Windows 7, 8.1 or 10130 Windows Install.