What NaCl seems to do is to restrict the instruction set, so that it can be verifiable, at the expense of requiring a special compiler.
However, honestly, that seems an absolutely unnecessary complication, since it's possible to just run any arbitrary binary code is a separate process, isolated by OS functionality that only allows a very limited set of system calls (e.g. seccomp in Linux).
Windows might not provide a similar system call limitation feature, but it should be very easy to implement in a Windows kernel driver.
That would increase performance, allow to avoid having to use special compilers, as well as allowing trivial portability to any CPU with an MMU and built-in protection features.
In other words, IMHO this technology is horribly complicated without any real reason for that (since hardware offers the same security, much better), although it's somewhat academically interesting to see how instruction sets can be restricted to be made verifiable.