No sir, I have never really thought about that. But I have seen how handy it could be.
Think about this: If you can stand the helm seat being raised an inch or so, you could easily put a sideways slider under it, and fashion a quick and simple to deploy knee brace to hold it up. You would be limited to getting on the thing from one side only, but you could sure get it out of the way in a hurry without any readliy visible signs that your boat has been modified (modified = upgraded if you do it, dorked out if I did it).
Here's another method. Mount the seat on a rotating arm which is attached to the seat in the center with a pivot and to the box beneath the seat at the inboard edge with another pivot point. When the seat is in the standard position, have the unsupported end of the arm rest on a piece of plastic with a detent (or latch if you're the nervous type). That would make the seat plenty solid and look like nothing is different. When you swing the seat out into the aisle, it would end up one full seat width to port (or more/less depending on how long the arm is and how far off center it is attached to the seat). Have a two piece hinged (or telescoping) rod that stores on the side of the rotating arm but can be lowered and extended to make an angle brace from the seat to near the floor. That would hold the seat rigid and in place when moved out.
The simplest would be just what you suggested - a bar stool in the aisle. But you have to get it out of the way sometimes, and also stow it. The aisle isn't much, but it is the route to the PortaPotty. You could make some gizmo and sell it at next year's boat show, no doubt.
Sheesh - I wish you wouldn't post this kind of stuff so close to bedtime. I'll be awake half the night trying to picture how that might work. You are a mighty sharp gentleman and a natural at thinking a little outside the whatever (or so I've heard), so maybe you ought to spend a few hours late at night thinking about this, too.
Man, you're gonna be famous. Could be the next iPod.
Sorry I get so carried away, and if all you really want is something to lean your butt against, then just ignore me.