I realize that most skiiers don't realize this, but...it just so happens that most ski lifts have, have had, or could have lap bars on them (I realize that on many, they have been removed).
I see no reason why a lap bar couldn't be connected to the usual overhead pivot point. Do away with the armrests on the sides of the car and integrate those into the lap bar...so you pull the bar down and it serves as both lap bar and armrest. Setpoint has a pretty good design, but I don't know if their car is designed in such a way that it will secure an inverted rider. It wouldn't take a whole lot to do that, though...the trick is to make certain that the body is forced to remain in a sitting position (you won't come out unless you straighten out). That can be accomplished by making the seat a little deeper, and possibly by making the lap bar fairly deep (front to back).
Dave has a sudden thought. You wonder how you might attach a lap bar to an inverted coaster seat? Try this for inspiration: Make a visit to your local Toys-R-Us store or any other store that sells baby goods, and take a look at the kiddie seats they sell for cars. Many of those have lap bars on them; notice how they handle the mounting (ignore the fact that most also have shoulder belts on them... :) ).
--Dave Althoff, Jr.