Fortunately I had just swallowed my water before reading this. Seriously though, the idea isn't shocking in itself; I'd just be worried that it might become a source of temptation to the producers to add more musical numbers, even if this particular scene had a good number behind it. (The way I feel about a musical number here would really just depend on the quality of the song. There's a huge danger of the scene being made ludicrous by it, however, so it may just be safer not to do it at all....)
The closest thing I can think of to this scene in terms of energetic musical numbers might be Step in Time from Mary Poppins. The somewhat crazed behavior of the chimney sweeps as they leap about, twirling people around and disrupting things, reminds me a little of the overthrow of the Telmarines in this scene; frenetic, mischievous, yet positive.
Maybe I'll think of a better comparison later, but that's the closest thing I can think of at the moment.
Also, would it be better to have the characters sing in the scene, or have a singer in the background?

I would be very opposed to a musical number a la Disney movies... where everybody spontaneously breaks into song and dance. It's not "realistic".

Having said that, there are moments in Lord of the Rings where people sing songs, either the hobbits singing in a bar, or when Pippin sings to Lord Denethor, that are very natural and add a HUGE amount of beauty and culture to the movies. I would be completely on board with this type of musical number.

OK, please restrain yourselves from spitting whatever you're drinking all over your screens.

Too late.

HAHAHA.
In all seriousness, I don't think it should be a musical. However, I do think you're on to something.

I would be very opposed to a musical number a la Disney movies... where everybody spontaneously breaks into song and dance. It's not "realistic".

Having said that, there are moments in Lord of the Rings where people sing songs, either the hobbits singing in a bar, or when Pippin sings to Lord Denethor, that are very natural and add a HUGE amount of beauty and culture to the movies. I would be completely on board with this type of musical number.

Fantasia, you took the words right out of my mouth - right on the money there.

The same thing could arguably be said for the creation of Narnia - whenever Netflix gets around to adapting MN.

I am so happy this thread is awake again, btw.

I also agree with Fantasia; I don't think a musical number would do, but I think a song would actually be splendid for the Romp scene. Honestly, there's a good argument that it's necessary for a faithful adaptation:

At a little town half-way to Beaversdam, where two rivers met, they came to another school, where a tired-looking girl was teaching arithmetic to a number of boys who looked very like pigs. She looked out of the window and saw the divine revellers singing up the street and a stab of joy went through her heart. Aslan stopped right under the window and looked up at her.

And so at last, with leaping and dancing and singing, with music and laughter and roaring and barking and neighing, they all came to the place where Miraz's army stood flinging down their swords and holding up their hands, and Peter's army, still holding their weapons and breathing hard, stood round them with stern and glad faces.

It wouldn't be the Romp without songs and music and dancing. Not a Broadway-style performance, something put on for show, but pure revelry; the reunion of old friends and the return of Old Narnia and all the attending joy.

Thinking about it, the "yodeling cries" of the the Greek characters during the Romp — euan, euan, eu-oi-oi-oi — might do for a song, or a piece of a song. The Irish choral group Anúna has a song called Fionnghuala that comes to my mind when I try to imagine how that particular scene might sound.

I've actually grown quite fascinated with the idea of having musical numbers in movies/media that wouldn't be expected to have them, so I actually kind of like this idea right off the bat. (And no, I don't mean those comedy episodes of TV shows which have everyone burst into a parody of musicals, I'd like to see it played straight. But I couldn't tell you an example of what I actually am thinking of, partially because I'm not entirely sure what I want. )

What I'm imagining is kind of a fusion of an in-universe song and a musical number. So, the revellers are singing an in-universe song (like fantasia suggested), and the background music harmonizes with the in-universe song. As the Romp continues, there are sort of cutaways to the vignettes (such as the school boys getting, possibly, turned into pigs ), and while those scenes would not be sung, the pacing and choreography are still set to the music. The song may not literally be continuing during these cutaways beyond background music, but it's almost like the music and the joy are so infectious that the rest of the world can't help but be caught up into it.

I think the closest example I can think of to what I'm imagining is the Bonjour opening to Beauty and the Beast, how there are several points where the music shifts to the background while there are spoken conversations. But what I'm thinking of would make the spoken conversations (and actions) more closely connected to the actual music, if not explicitly so.

I'm going to take this opportunity to admit that I am not a musician, I'm one of those people who comes up with crazy ideas regardless of whether or not they'd actually work.

I think it doesn't even have to be a full musical. I can almost imagine Miraz singing his rendition of "Be Prepared" from The Lion King; he and Scar have such similarities.