So, scripts in the Landscape client run as root within the confines of the Landscape client snap.
As this snap has the snapd-control interface it can do a lot, but it is still confined. Granting the ability to break out of the snaps confinement would make it a root-kit which would have very serious consequences.
The content interface could help here but has the difficulty that landscape-client would need to be hard coded for each snap it wants to interact with.
The solution is confdb. Confdb would allow the mosquitto snap to expose a view onto the underlying configuration file that could be shared. It is still experimental, but it is the intended way to expose configuration for the future.