Currently there seems to be no way for a shell extension to register a new keybinding alongside the built-in mutter keybindings. This blocks a number of useful extensions that need more (or different) keybindings to what mutter has built-in (e.g a tiling window extension, which I am currently working on).

Currently I believe the only way to add a keybinding is by adding it to mutter's source code (all-keybindings.h), adding a (NOOP) handler to it, and recompiling. This obviously makes distributing such extensions implausible, as combining multiple such extensions would require merging & recompiling on the user's side according to their particular set of extensions.