>Now, in practical terms, this means that, given a MZ twin who died recently of natural causes, we could obtain an estimate for ourselves,

Uh... am I misreading your comment, or are you suggesting that when your identical twin dies of non-accidental death, you can be pretty sure you're about to croak in the next wee days or weeks yourself? Very difficult to engineer that alarm bell (you either have a twin, or not), and too damned late to matter.