Out of pure curiousity since I read a lot about AGS' limits I was wondering what about Multi-scrolling.
I know that there is a plug in for parallax but I know nothing else.
AGS doesn't have any built-in parallax scrolling functionality. But it can be emulated in a number of ways - using RawDraw or objects for the different layers. As you say, there's a plugin, but you can also check out the Easy3D module (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=26130.0). There was another module, but as far as I recall it only supported foreground objects.