Sorta reminds me of the i3lock screen locker. It shows an incredibly confusing circle UI where every keystroke randomizes the position of the sector on a circle, with no explanatory text on the screen (^1). To new users, it's not clear at all that you are entering your user password or even that it's a screen locker at all, because it just looks like a cryptic puzzle.

Of course, once you do understand that it's just a password prompt, it's great. Completely confuses the hell out of any shoulder surfers, who will for sure think it's a confusing puzzle, and eventually they will get rate limited.

^1: Example of it in use: https://www.youtube.com/watch?v=FvT44BSp3Uc

Now that you mention i3lock, if sudo showed a symbol changing with each keystroke, it could show it's working (not frozen, accepting input) without revealing the length, similarly to i3lock. I've seen ascii loading spinners from package managers by changing between slashes and hypens and such. Something of that sort would probably do the trick.