Nice idea, I haven't seen a suggestion like this before!
Though, to be perfectly honest, I actually wouldn't want this. I just feel like certain fonts would be too hard to read (it's hard to read a lot of posts even on the forum too, especially if you consider font colors!) and with a mix of tons of different fonts in the chat it would get really mixed up and make the chat look a bit wonky, especially since there's so many different sized fonts (as some fonts are more narrow while others are huge, even if they are at the same size). But if there was a way to toggle it I'd be fine with that, I just think if it were on all the time it would cause some troubles.
But I do think this is a nice idea though, and it'd give in-game posts some more uniqueness c: As long as it would be toggleable I think it'd be an interesting addition to the game (assuming we had the source code and blah blah)