Circular, I think your idea of having a trusted person to help decide "when" is key. Finding the right person would be critical, however, as he/she could feel responsible for the suicide. From the tone of Sandra Bem's obituary, I sense that she had people around her who understood.
I don't have children, either. It is just my husband and me. We are both planners, so we will document a plan for my future if I get AD (he's a 3/3, so we're counting on him not getting it). He is definitely my "trusted person" in all things, but if he goes first, we want the nieces and nephews to know my wishes.
When I started to write this post, I got sidetracked with the following paragraph. I almost deleted it but then left it because it is something I have mulled over for the last 4 years and finally put into words.
Long before my mother was diagnosed with AD (but after my grandmother was), she speculated that many accidental deaths of dementia sufferers were actually suicide. She also followed Dr. Kevorkian's work and we had many discussions about physician assisted suicide and euthanasia. It was all speculative but I am certain she considered her own options, should she ever get AD. Ultimately, I think my mom prioritized her faith and concern for others before her own well-being. First, could she claim to have faith in God's plan for her and then commit suicide? (That was one "what if" we never did resolve: Could God's plan ever be suicide?) Second, what effect would her death have on her family? She knew I would understand, and my brother likely would, but my sister and my father would be absolutely devastated. She would also consider the impact on everyone from her grandchildren to, say, the paramedics called to the scene of her death.