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:
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.
I agree that the first universe is better, but I'd be way too busy mourning the death of the planet to mourn the interval between those two outcomes if the planet was actually dead. You could call that mental accounting, but isn't everything?
So I recently found myself in a bit of an awkward position.
I frequent several Facebook discussion groups, several of which are about LW-related issues. In one of these groups, a discussion about identity/cryonics led to one about the end Massimo Piglucci's recent post (the bit where he says that the idea of uploading is a form of dualism), which turned into a discussion of LW views in general, at which point I revealed that I was, in fact, a LWer.
This put me in the awkward position of defending/explicating the views of the entirety of LW. Now, I've only been reading LessWrong for <10 months, and I've only recently become a more active part of the community. I've been a transhumanist for less time than that, seriously thinking about cryonics and identity for even less, and I suddenly found myself speaking for the intersection of all those groups
The discussion was crossposted to Felicifia a few days ago, and I realized that I was possibly out of my depth. I'm hoping I haven't grossly misrepresented anyone, but rereading the comments, I'm no longer sure.
Felicifia:
http://www.felicifia.org/viewtopic.php?f=23&t=801
Original FB Group :
https://www.facebook.com/groups/utilitarianism/permalink/318563281580856/
EDIT: If you're commenting on the topic, please state whether or not you'd mind me quoting you at felicifia (If you have a felicifia account, and you'd prefer to post it there yourself, be my guest.)