I believe someone at 343i (maybe Stinkles? If I'm remembering right it was quite a while ago so I'm not sure) might have lightly hinted it was possible, but that's it. Unless I missed something.
As for how it would/could be integrated, theoretically MCC could import H5's contents from the HDD and from there, yeah, it would probably just be extra tabs everywhere and maybe some new cross-game matchmaking/campaign playlists.
Regarding MCC vs. non-MCC owners, they could include all of Halo 5's matchmaking playlists within the MCC UI. If you choose one of those H5 playlists you'd use that game's matchmaking system (making MCC essentially just a UI shell on-top of H5) and be matched with anyone playing H5, either through the game itself or MCC. If you were to choose a MCC cross-game playlist with H5 maps, you'd use MCC's matchmaking system and exclusively match with MCC-based players. Or something like that.
Granted all of that seems like it could become extremely complicated on the technical side very quickly, and even if they had planned for this kind of integration, it's probably been shelved or canceled outright due to MCC's issues. It's been half a year and vanilla MCC still has issues, I doubt they want to tackle something that complicated on top of continuing to iron everything out. And 343i/MS probably want to distance H5 from MCC if they can. So yeah, I'm not expecting it at all.