I have some reservations about this.
What you're describing seems like it could work, but the gap in the gas-in opening (your gas-out in this setup) is small, if that gets plugged, or the tap gets closed, this thing will go critical very fast.
The thought of an exploding sanke keg scares me, and there's really no reason to go this way. I'm sure you know about the kits that exist to ferment in a sanke, and orange carboy caps work fine to cover the top if you don't want that expense.
Good luck with however you go, and stay safe.