Is there any actual incompatibility that prevents building an Objective-C runtime for NaCl, or is it simply a matter of ObjC not being included by default?
I've been thinking (warning: pure speculation ahead) that I could target my binaries to LLVM IR and then use the highly-experimental PNaCL. But who knows how long it will be before that is viable.
This is pure speculation, but its possible that the runtime does some tricks for performance (possibly in the method dispatch routine) that the NaCl sandbox doesn't allow.