[strcat]

NaCl doesn't imply proprietary any more than a JavaScript engine does. The downloaded code is bytecode at an abstraction level a bit below C but quite a bit above assembly. It's not substantially different from a freedom perspective than code compiled to asm.js or just minified JavaScript. Both are usually proprietary and require reverse engineering work to decipher.

[vezzy-fnord]

As it's not the "preferred form of the work", I think it is proprietary - same way any object/intermediate or obfuscated code would be.

[rockdoe]

>NaCl doesn't imply proprietary any more than a JavaScript engine does

http://www.gnu.org/philosophy/javascript-trap.html

[diamondlovesyou]

> The downloaded code is bytecode at an abstraction level a bit below C but quite a bit above assembly.

You just described PNaCl, not NaCl. NaCl is architecture specific and already sandboxed (ie the SFI part of NaCl), whereas PNaCl is not.