I was skimming through Marlin PRs and found this pull request on implementing a differential extruder:

This reminds me of when I had a similar belt idea for a cleaning roller in the SecSavr Suspense (which is currently named #SlimeSaver [gd0105]).

I'm thinking that this could allow for direct-drive extrusion for 3+ input hotends without the drawbacks of other methods such as a semi-stiff stick or mounting many heavy motors on the end effector.

If I understand correctly, I think this would even be Core XZ compatible, allowing all steppers to be in the base of machine like the Ender-3 V3.