You are correct. There isn't a way to limit it to a single response group anymore. You can scope RBAC based on Lync Site and OU (OU doesn't apply to RGS) but that isn't going to be as granular as you want. I know Microsoft has heard this
complaint several times and I wouldn't be shocked if things changed in the future but currently it's not there.
Richard
Richard Brynteson, Avtex, Lync MCM