So far XPCNWs haven't been terribly precise about this, because we're always calling into C++, which is going to unwrap everything anyway. Bug 563106 needs stronger guarantees which means being more precise.
I think we plan to do this after bug 568671. Otherwise it's redundant work.
If that starts taking too long though I can do the redundant work, which isn't a whole lot, I think.
The XPCNativeWrapper re-implementation in js/src/xpconnect/wrappers takes care of this. (It's called XrayWrapper<JSCrossCompartmentWrapper>.)
This is not exactly FIXED in the usual sense but I'm marking it RESO FIXED anyway because there's no point worrying further about it.