Terrible idea, you will need to put holes in the side of the case for fans, in which case you will need an intake hole, and unless you attach some type of air filter to that, you may as well buy stock in a company that produces canned air to clean computer components.
Not to mention that if the motherboard or what not gets too hot, the legos could possibly melt, or worse, the glue could melt and get all over your PC. Also, how are you planning on suspending a motherboard in the middle of the case so that air may flow between all of the parts, cards, hard drives etc.?