A few months ago I created/worked on a project that involved separating PNaCl (the portable native client) from the Chromium project, rewriting parts of it for use as a generic JIT compiler, and building a small PoC distro around it where all of userspace were PNaCl binaries except for the loader/JIT (which was a regular old architecture-specific ELF that existed read-only in memory and loaded by a slightly-patched Linux kernel) Only PNaCl executables were allowed to run in userspace, which allowed for secure execution of user binaries thanks to the sandboxing features of the modified portable native client. However, another benefit of the PNaCl is that binaries are compiled to a performant architecture-agnostic IL (similar to LLVM's IL) and then quickly finalized by the PNaCl loader into a regular architecture-specific binary and loaded into secure memory when they are started.

This meant, in a nutshell, I could have a completely architecture-agnostic root filesystem, with the only architecture-specific binaries being the kernel and PNaCl runtime, which sat in the initial ramdisk.