Largely agree with this. I do think Kendra -- at least the Kendra we meet in What's My Line? -- would agree with Giles about sacrificing Dawn to save the world, so Buffy's reluctance to tell her would make sense. But equally, Kendra would (I like to think) "remember" meeting Dawn and be very fond of her, and probably she'd be at a point in her own character arc where she wouldn't be quite so sure about what the right course of action was. And of course either way Buffy not trusting her to make the right choice would hurt.
As well as the season finale apocalypses though, there are at least two other potential world-evening events where Kendra could show up: the Sisterhood of Jhe trying to open the Hellmouth in The Zeppo and the Vahrall demons trying to do the same thing next year in Doomed. I think it would neat to have Kendra come back to Sunnydale for one of those, and I think the best choice would be The Zeppo.
I'd keep the whole thing from Xander's perspective, as in canon, which saves the need to explain exactly how Kendra and Zabuto found out about this one. Just have Xander walk in on Buffy having a big strategy meeting with Kendra and Faith at one point, have him wonder when Kendra arrived, then get ushered out to get donuts or something. Maybe another appearance during the post-battle scene at the end where Willow praises some heroic feat Kendra did off-screen, too. (Or have Buffy be the one to offer praise, and have a quick shot of Faith looking irritated.)
I wouldn't want to have Kendra making a cameo appearance mid-season to stop the Hellmouth opening a regular bit though (and I don't know that Kendra would react well to meeting chipped!Spike or Riley) so I guess we'd want to have some big problem happening over on her own show that clashes with the events of Doomed. Have a throwaway line where Giles mentions that they couldn't get in touch with Kendra, have Riley brightly ask "who's Kendra?" and then have Buffy awkwardly change the subject.
The other season finale that I think Kendra should be connected to somehow is Restless. In the world where there is no Kendra the Vampire Slayer spin-off and Kendra stays dead, I think (as I've said before) that it should have been Kendra, not Tara, who acted as the voice of the First Slayer. But in a world where Kendra is alive, I don't think that works quite as well.
So, instead, I'd first add a flashback scene earlier in that season of Kendra the Vampire Slayer where we get to meet the Slayer who died just before Buffy was called. (Not the one from the tie-in novels: a new character.) Then I'd have her step in for Tara over on Buffy (with Buffy not knowing who she was, but maybe figuring out from context that she was a former Slayer). Meanwhile Kendra could either find out about the spell secondhand or, maybe simpler, have her own dream featuring the First Slayer? Maybe one that ties into the longer series arc of Kendra's previous death and resurrection? Some variant on the 'death is your gift' bit from Buffy, somehow? Not sure of the exact details.
(Also, a final thought. It would be horrible for both actors in practice, but a fun thing to do in principle would be to have the First Evil appear to Buffy in Season 7 as Kendra, and meanwhile have the First appear to Kendra over on her own show as Buffy...)
