In the same direction, a /say command would be useful too
I thought it was possible to prefix the msg with a weechat vanilla command to make it pass through the command hooks but it seems we instead have to implement this on the plugin.
I think of a simple "/matter echo foo 5" which will send the command "/echo foo 5" to the mattermost server. I hope that the process to send a command is the same as the one to send simple msg.