There has been late submissions in all our past jams, LRJ specifically usually about 10 or so. And like your case, we often know about them in advance even if it's kept relatively private. Any time late submission is generally no problem, but keep in mind the later you are the less ratings you'll get, and you usually need >10 to even be ranked properly.
To submit late, please contact me on Discord (preffered) or Twitter. You'll need to have uploaded your game, then just send me a link to the game page. I can then submit your game to the jam and send you back a confirmation link.