Hm. I think worrying about whether something can "harm" a dead person carries much more semantic baggage, so the key ideas will probably be different.
Good point. I think the main similarity derives from a specific understanding/definition of harm that holds that harming another is acting counter to another's preferences, in some sense. In that way then, it's similar to (the OP's trouble in getting his interlocutors to understand) preferences being sustained after one's death.
More than once, I've had a conversation roughly similar to the following:
Me: "I want to live forever, of course; but even if I don't, I'd still like for some sort of sapience to keep on living."
Someone else: "Yeah, so? You'll be dead, so how/why should you care?"
I've tried describing how it's the me-of-the-present who's caring about which sort of future comes to pass, but I haven't been able to do so in a way that doesn't fall flat. Might you have any thoughts on how to better frame this idea?