Honestly, there's not really a sensible place to put "Airlock" at all, unless they suddenly decided to do a special edition re-release of "The Time Meddler." That's the only one that really comes close to the same chronological position. Plus, "The Time Meddler" is the only complete surviving story to feature that particular companion pairing of Vicki & Steven.
Have they announced yet what they're going to do with "The Underwater Menace, Part 2"?
Personally, I kinda want them to do a special edition of the "Lost in Time" collection. They need to add something, whether it's live action linking narration or even just text recaps to fill in the gaps of the missing episodes.