Huey, it's not just the weight. For that seat they don't have to provide catering, baggage handling etc and nor is there any passenger tax associated so I reckon it'll be a money spinner.
I don't believe it's a stupid idea. I might not be prepared to pay for it but I bet you there are people that are, and that's all it takes.
As for managing selling the seats it's easy! Reserve the skycouch section for those willing to pay for it upfront, when or if the rest of economy sells out then just start selling these as individual seats if they aren't already sold. It's not rocket surgery
As for 3-4-3; yeah not so happy.