there´s plenty of hide hacks for different bb systems, i´ve only seen one for ubb though, the simplest kind that just hides the text between 2 hide tags until you reply... it doesn´t seperate mods and amins from posters though... and i have no idea who wrote it... i´m just saying its very possible to do...
I've seen a lot of them on vbulletin boards. (they use a picture to cover the data that's supposed to be hidden) You could use anything off course But there needs to be something so you can see that the hide function is used inside a topic.