Hello,
Mattermost newbie here (we'll be setting up a server in our office soon) and I have been looking for this feature, so I personally would welcome it very much.
As for the options, using a #hashtag would be more convinient, but if this too complicated to implement, option 1 would also be fine in my opinion. This will be especially useful for users who are not very tech-savvy.
Thank you and best wishes