A few things to clarify:
Historically, my father loves surprises. My mother doesn't like them as much, but only when they fall into a certain category-like she would hate a surprise party for herself, or hate a surprise that *she* felt like put her on the spot/gave her more work to do, even if the obligation was all in her head. (For instance, if we had a surprise party at our house, she would feel obligated to help clean up after/entertain everyone and she wouldn't really enjoy it.)
Knowing this, I don't feel like my brother's coming home falls into this category. We have a well-established routine for Christmas, and we wouldn't be doing anything differently if I sneaked my brother in. His room is the same as when he left it, so he'd have a place to sleep, and the quantities of food prepared won't be different in his absence.
I agree with people who say that "I'm going to Starbucks on Christmas Eve" is a flimsy excuse. I may just level with them and say "I am going to get your surprise, I couldn't get it until now" if it comes down to it.
As far as the cliche goes, I know that there are a ton of videos about it on Youtube or people doing it in public. But this wouldn't be a big to-do because a) we're not filming it (unless we did @ my grandparents' house for our personal enjoyment) and b) he's only been gone since August. When he left we'd well established that there was no chance of him coming home for the holidays and he might get leave in March.
Now there's a good chance that he *might* get off for Christmas. I have to qualify that (even though I said "great" in my first post!) because he just texted me and said that he's been picked for some sketchy something-or-other program and might not end up coming home. He tells me a LOT of stuff that he wouldn't ever tell our parents because they would just fret over it. I feel like it isn't months of lying in this case. Even if he does get to come he won't know 100% until his flight is booked in December, so we've already saved our parents/grandparents at least a month of the roller coaster of yes/nevermind no/yes/wait let me see what this program says.
Even if it is a cliche, do you think that our relatives would think "Oh, this again" if they saw him? Like, would some of the "novelty" be worn off of it from so many videos/news snippets on it? Again, we're not trying to stage anything or make a big fanfare other than plopping a bow on his head or something.