I believe that would be useful - I could see several instances where for whatever reason we would tag a post ('er, or is it "message" now?) differently than a Topic ('er, or is it "Subject", which the gio GUI confusingly uses interchangeably?)
Are you thinking of being able to choose from the list of hashtags available for Topics? Or a separate list of msg tags?
Mark has asked the group to not get into what's possible/not possible to implement (his call to figure out) so ignore those type negative nay sayers.
On the positive side instead, it would allow for a more useful search of messages (opposed to the woefully inadequate, 17th-century, non-boolean, word/non-phrase search we now have to operate under.
Hmmm, Y'ah know - thinking aloud here - there's nothing preventing a group from adopting a set of message tags and just entering one (or whatever) as the 1st line, or last line of the message. If tagged with something very unique -- like ## as J said, or TAG-your_tag, or enclosed in %your_tag% -- you could search and more easily retrieve messages. I'm liking this.