I did a clean install of Win 8.1 RTM on a Lenovo Ideapad that came preinstalled with Win 8 and has the key in the BIOS. I inserted an ei.cfg file into the iso on the USB flash drive per floepie above:

Booted to the USB flash drive and proceeded to install, selected the Core version to install to match what came with the laptop, skipped the key entry and then finished the install.

To successfully activate I had to retrieve the BIOS key using instructions from this post and manually enter.