The reader would just work it out intuitively in passing, I think, especially as likely there will be a coloured box at the top of every topic. The writer just types # so wouldn’t have to know either.
I quite like the idea of moving away from displaying the hash symbol for multiple things, and would probably vote for the proposal above – one/two colour (category) box and chat icons, one-colour tag icons. Though the two-colour chat icon would take some getting used to.