Late 2012 iMac not seeing Bootcamp 6 update
According to this thread on Apple Support Communities, the version number "6" only shows up in Windows, or in XML files on the Mac, if you go digging.

From what I understand, you should be able to, without any problems, use a Windows 10 ISO with the 5.1.4 version of Bootcamp Assistant, and it'll be smart enough to download the version 6 drivers behind the scenes.