Hi @jwilander - thanks for the reply.
I've logged a feature request here: https://mattermost.uservoice.com/forums/306457-general/suggestions/13380846-add-auto-link-functionality-e-g-to-create-links
Note that I came across this project, which looks like it might do something similar to what I'm asking about: https://github.com/woutervb/mattermost-jira-info
I can't seem to find any instructions about how to actually integrate this into Mattermost nor how it actually works in practice, so I'm not sure if it quite does what I'm after, but it looks like it's using a web service to modify the message text when submitted, in which case a variant of this might solve my problem.
What do you think? Is it doing what I think it is?