Since this has now moved to a request for system-files access can the topic of this request post please be updated accordingly so that it is clear exactly what is being requested?
Rather than having a snap need have to have write access to this location on the host system and install a file via a hook etc, instead ideally there would be an interface which allows snaps to use this in a more controlled manner, and which ensures that the resulting native-messaging file only specifies launching a binary within the snap, under-confinement etc.
However, until such an interface exists, +1 from me for granting write access to system-files as requested.