I think this would be a cool idea...But you'll know when someone's away if they set their status to "AFK"

This feature was in XI and in 1.0...it would show an away icon next to their name and in the search feature. But the difference between them was that in XI if you tried to /tell the person it would auto reply with a system message that the player is afk, like mentioned already.

I like your idea though! I think that would be cool. It'd be like your character is a temporary NPC. Maybe if you had a bazaar open, you can set a message that when people would walk by you, they see the bubble message (maybe you're selling certain things while you're afk) and your character can be selling stuff for you while you go walk the dog or bring a friend somewhere! XD That'd be great. I'd like that.