[rogerh]

I believe you are putting too much thought in hardware. I built this window seat, in two of my dormers, from a knee wall and 3/4” lumber covering.
After I built the knee wall, I screwed ledger boards, to the walls, around the dormer. I built a 3/4” subframe around the dormer seat opening, about 1/4” to 3/8” smaller than the window seat door seat. The frame will rest on the ledger board. The frame along the bottom of the window needs to stick out from the wall about 4 to 6 inches. That way, when the window seat is open, it will stay open. You can use about any kind of hinge you like. When the seat/door cover is installed, it should be 1/4” larger than the frame opening. That way there should be a 3/8” ledge around the window seat opening.

 

[rogerh]

Why must the top be one solid piece? That's a recipe for an amateur outcome if I ever heard one. Do a neat & tight job with permanent installation of triangle pieces on both sides. You'll have nice big center rectangle to lift with a piano hinge at the back. With the top raised there will be good access to the entire storage area. Work carefully and the space between the center and side sections can be 1/16th inch and look totally professional. Your desired approach, IMO, will fail.

Just because someone thinks outside the box, doesn't necessarily mean they will fail. I hope his idea works and he returns with an update. That is what draws me to this site, is learning something new.

 

[rogerh]

It's a problem of geometry. No amount of outside the box will change that (assuming the top must remain 1 peice, and must be hinged at the rear)

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This is thinking outside the box and not letting geometry get in the way.