I'm by no means an expert here, but I'm not sure if a string retainer bar is really needed here. These headstocks are angled unlike F style headstocks giving the strings a decent break angle behind the nut. None of the Ibanez models I've seen have string retainers. Looking at my Ibanez S670 just now, all strings have the exact same angle behind the nut (except the low E but that's only because it's been wound poorly). Same deal looking at my son's RG and his IB-7 kit build.
I think you'd also have trouble fitting a string retainer with a truss rod cover in place. There's not a lot of real estate around that low E tuner.