To your point, you will not be prevented - but any patent that you would attempt to file would be a derivative work built upon any patents for Lego components included in your designs.
Additionally any invocation of the Lego trademark in association with your products would also be subject to legal limitations and compliance.
Please note of course that I am nothing like a lawyer and I offer at best a layman's meager understanding of US laws in this matter.
Wait! This isn't a Parachute, this is a Backpack!