NativeClient supports ARM (http://www.h-online.com/open/news/item/Google-s-Native-Clien...).

You would have to compile multiple versions to support multiple architectures, but the implication that NaCl is fundamentally tied to x86 is incorrect.

NativeClient's use of segmentation on x86 is just an implementation detail of how the sandbox is implemented on that architecture, AFAIK. NaCl doesn't fundamentally require that any target architecture support segmentation.