Win 8 RT only runs the silly Metro apps under an ARM architecture. The Desktop and therefore Desktop applications won't run. The only answer that I can see would be for Simon to write a completely new yWriter Metro App that mimic'd the real yWriter. Then he'd get requests for an Android (linux) version and an Ipad IOS (FreeBSD - Linux again) version.
Personally (and it's only a personal opinion), I think more a effective alternative would be to use a distraction free text editor on a Tablet (they are available for just about every flavour of OS now and most are free) and transfer one's writing back into yWriter back at the ranch. Me? I use a Samsung netbook (with 2GB RAM & Win 7 Home Premium installed) when out and about as it fits neatly into my manbag, is pretty well as convenient as a Tablet but has a proper keyboard rather than the flat bit of plastic that comes with the Surface (and is likely to give most proper typists severe cases of RSI). Best of all, it runs yWriter without the need for MONO, VM or any other emulation.
As I had it drummed into me in my years in the army - "Use the correct tools, correctly." :))