Hrm, actually… I don’t know if this is helpful, but you can configure Discourse to always email you when you receive a Message. It should be possible to implement the functionality you’re looking for outside of Discourse, and then simply use email to get messages in and out of the forum software.
I quickly googled around and came across this guide for Gmail, which includes using canned replies that automatically get sent based on filters (e.g. the incoming email features particular phrases);
https://flashissue.com/blog/2016/06/15/create-auto-reply-gmail/
If you use a different email provider, they likely have equivalent features.