Even if you did find a ventilation shaft big enough to crawl through, it would still be a terrible idea. First, they're made of hollow metal. Banging around in there would be incredibly loud. Second, they're not really designed to hold up the weight of an adult human. You'd just destroy it if you ever put your weight in a spot that wasn't supported, and you can't see where it's supported from the inside.
Sadly no. The screws hold the sections they're made of together. The supports don't always have to be on those joints though. They tend to be. It's better to support a joint when you can, they just don't always have to be.
