Yes.
I use Shell in a Box
With a root account, I can access to the config.json file in yunohost/apps/mattermost/conf/
I modified the file with the Nano editor.
I tried to see with the Zoom Plugin, but it is not the same thing :).
I agree with YNH, perhaps this does not work with this implementation.
See a request here: Collabora Online (LibreOffice) How to get Macros and BASIC IDE (Coolwsd.xml) - Support apps - YunoHost Forum
Thank you for your time.