I saw contradictory statements on this matter. I tried it and was unable to install win8.1 x86 in UEFI mode to a hyper-V disk (generation 2), setup did not even start - this is expected behavior according to microsoft. Neither could I install it to a real disk when UEFI mode was used to address the USB setup media.
Has anyone managed to do it? Or is the info here https://social.technet.microsoft.com/Forums/windows/en-US/9b8b18b3-df53-4f00-a584-1b1617b57ea9/windows-7-32bit-uefi-installation?forum=w7itproinstall
"Windows 8 x86 was made to support UEFI" plainly wrong? I even saw people claiming they installed it in UEFI mode, but I don't trust those anymore since it did not work on two instances here (while x64 of course worked and x86 in non-uefi worked, too).