Fretless mode for piano/custom overlay

Would love to be able to switch my keyboard or custom overlay to be fretless, possibly with just small lines indicating the perfect spot. I tried doing that with “Absolute X” switch but the range on that is way too high. If I could define my own “absolute X” surface with for example only one octave that’d be nice.
For bonus points, it could have a more advanced rounding mode, with quarter/eighth tone option as well as different smoothing algorithms.

Actually, I’m not sure how well it would work with piano overlay, because the keys are laid out unevenly. So it would have to be a custom overlay… which isn’t really possible within the app as far as I know, and I don’t have the knowledge to make something like that myself.