The safety question is if it would fail without any warning. Try riding it without fixing the quick release. If the bike just rattles then you will have time to stop and tighten the QR, if not it might be trouble.
I think the forces on the bike would tend to push the crossbar on top together, and pull the downtube apart.
You might drill through the tube walls and stems and insert a 4 pins as a safety. The drill holes will weaken the tube. I wonder if drilling vertically or horizontally would be better?
I think it is kind of clever. My guess is you will need to adjust the QRs often.
I once thought about using plumbing couplings.