I’m not aware of anyone currently working on this.
At a high-level there are a couple of different ways to handle additional axes - the simplest is to alter the orientation of the toolhead in commands and movements separate from normal XY moves (so called “3+3” control) - the more advanced mechanism provides simultaneous XY and tilt in a single move (so called “full 6 axis” control).
The former (“3+3”) may be possible to implement today with manual_stepper config sections (see docs/Config_Reference.md for details) and/or minor kinematic updates.
The latter (“full 6 axis”) would require significant work. Off the top of my head, it would involve obtaining a 6 axis slicer, modifying Klipper’s g-code move parser to support the additional axes, updating the toolhead look-ahead code to track and handle the additional axes, updating the trapq code to track the additional axes, and adding the kinematic math functions for the additional steppers.
Cheers,
-Kevin