I can understand why you can't do it yourself on-line (change in destination / point of turnaround would require a recalculation of the fare) most online booking tools are not setup to do this (very complex programming required).
Why not give their Res agents a call and ask them to do it for you. Dublin, without a London stopover (normally considered as being 24hrs) might even be cheaper than LHR.
It's been quite a while but my experience with the Exec Club is that they have no fare / res training. In one case, while I was still gold, I could see availability that they couldn't see. And this was intentional.