I wouldn't mourn them more than I'd mourn anyone else in danger.

I share and endorse-as-psychologically-healthy your general attitude to grief in this kind of situation. Both the broad principle "Would that be useful?" and the more specific evaluation of the actual loss in expected-deaths with existential considerations in mind. That said, I would suggest that there is in fact reason to mourn more than for anyone else in danger. To the extent that mourning bad things is desirable, in this case you would mourn (1 - p(chance of positive transhumanist future)) * (value of expected life if immediate cause of death wasn't there).

Compare two universes:

• Luna is living contentedly at the age of 150 years. Then someone MESSES WITH TIME, the planet explodes, and so Rocks Fall, Everybody Dies.
• Luna dies at 25, is cryopreserved then 125 years later someone MESSES WITH TIME, the planet explodes, and so Rocks Fall, Everybody Dies.

All else being equal I prefer the first universe to the second one. I would pay more Sickles to make the first universe exist than for the second to exist. If I was a person inclined towards mourning, was in such a universe, the temporary death event occurred unexpectedly and I happened to assign p(DOOM)=0.8 then I would mourn the loss 0.8 * (however much I care about Luna's previously expected 125 years). This is in addition to having a much greater preference that the DOOM doesn't occur, both for Luna's sake and everyone else's.