If you install Windows 8 Pro then add WMC and activate with WMC key you could then upgrade to 8.1 RTM this will give you a now activated Windows 8.1 Pro with WMC. Now once this is done take the data.dat file from that install and make a new ProWMC ISO using Windows 8.1 PRO RTM. This would let you install using the WMC key and install 8.1 with WMC also in 1 go no upgrading or anything and will be activated on reaching desktop.
Leaked other iso Windows 8.1 RTM Please and Please leaked multilanguage/ other language . Thanks Victor
Data.dat switch is obsolete and not needed for phone activation of the free WMC keys. Tested several times now without switching data.dat.