I was lucky enough to be invited by a friend to attend the media event with them. Yes Gringotts has a lap bar and that does include restraint against your thighs and shins. Could that restrict some sized people from not riding? Maybe yes. But I'd say it won't be something that will be as restrictive as FJ was when it first opened.
Honestly, the ride has to be safe and it is impossible to make a ride that will perfectly fit every body size, especially a thrill ride. The way Gringotts spins and jolts suddenly, I think the lap bar/leg bar system is the right way to go for this ride. Gringotts contains many spins and sudden jolts and with the open sides it is best to keep a secure hold on all extremities inside the ride vehicle. The restraints are not put into these thing for giggles but for a very necessary reason. Yes they could put in a ride that every body shape can ride and they did, the Hogwarts Express, but just like Gringotts comes with a height restriction for safety reasons, it also comes with a restraint system for safety reasons.
I know people like to think "it's not fair" that the ride doesn't fit all adults but logically speaking that is just not safely possible since there are so many different shapes and sizes unless you are talking a non-thrill ride.