A Modder over XDA forum claims to have discovered an exploit that allows to flash Windows 10 Mobile and Windows Phone 8.1 to any Windows RT device. The XDA user in question hasn’t exactly revealed the exploit he is using but it seems it is some recent “Secure Boot exploit” that he plans to use for it. He also claims that Windows Phone 8.1 is already working on Windows RT devices.

The exploit is for workarounding Secure Boot, and is applicable on all architectures.
A FrankenWindows between IoT and Mobile should be possible though, but I’m not betting on it

Many Surface RT users are offering to use their devices for testing of this hack given the abandonment of Windows RT platform by Microsoft.

We will keep it updated if more info about the exploit or Disk Images and screenshots emerge.