It seems like GRUB is as far as you gonna get...https://forum.xda-developers.com/windows-8-rt/rt-development/wip-secure-boot-linux-surface-rt-t3653848
Never trust headlines... I really thought someone would whip up a basic Linux ARM distribution but i guess no one attempted such as thing !They did however manage to disable secure boot.
However, it would help to know the hardware behind it. I guess you could try booting an upstream kernel with grub and compile your own userspace with buildroot but that basically requires, well, doing almost everything from scratch.
There's also WinEmu (now abandonned) that allowed running x86 WIndows apps on jailbroken Windows RT devices.Point is, make sure it's Windows RT 8.0 and not 8.1.