It is too bad that (from all appearances) Mordin, Legion, and Thane won't show up, but hopefully they get a mention or two.
I think 2 of the 3 are dead by the point where you're allowed to start the DLC...
Yeah, from what I hear, it starts after the Cerberus attack on the Citadel, so that's Mordin and Thane gone at that point. Although Legion could conceivably show up if you don't rush straight down to Rannoch after recovering it from the geth dreadnought. I guess it all depends where they decided to put in the hooks to main the story and when stuff becomes active.
I think I'll do a run through of this DLC with my Femshep first since that's the one that has everyone alive (everyone that can be alive anyways). The only other game I have available is missing Garrus after he took a rocket to the face on the Collector Base in ME2.