Emulating an x86 on an ARM like the Tegra 4 would probably result in a pretty unsatisfying experience. Apple was doing the opposite: Emulating a processor with worse single threaded performance and (arguably) a simpler ISA.
The alternative is a full port of Win32, which I imagine Microsoft considered and rejected. Even after they incurred all the time and development cost, ISVs would still have to at least recompile.
I think it has a nearly-full port of Win32, it's apparently not hard to disable the signing requirement and run cross-compiled Win32 ARM binaries and turn the Surface RT into a useful computer.
Not much commercial software but the regular open-source windows stack works.
I don't think it's a feasibility issue, because a fair bit of Win32 had already been ported to ARM from the Windows CE days. I had .NET apps that ran on Windows XP and Windows CE from the same binary. As long as you stuck to the .NET micro framework subset, it worked great and was actually quite performant.
I agree with you that this is technically a solved problem. It's not difficult to develop a system that lets apps run on both architectures without users having to think about this.
I'm talking about the management decision that only Metro apps can run on RT, which is such a stupid limitation that even Microsoft sees the need to bypass it to get Office on an RT tablet.
