Recently Nextcloud Talk implemented the capability to deploy bots that interact with chat rooms.
https://nextcloud-talk.readthedocs.io/en/latest/bots/
I wrote a custom component to use a bot to publish messages via service calls in Home Assistant. Feel free to try it out and report issues and ideas.
The next step is to make it possible to write commands in an NC Talk room processed by this component in Home Assistant, e.g. @ha open garage. So you can allow someone in your NC instance to open the garage without exposing the whole HA UI to him.
Update: In version 0.2.0 you can now react to messages sent in a room and do some stuff like open the garage door if somebody is asking.
This integration now supports two-way communication via webhook, please read the README!