Hi, I have for sale Schroth Quickfit for R53 driver side. Used only on 4 HPDE events before switching to Sparco seats which requires a different harness setup. This works with the stock seatbelt and and the stock seat and locks you in place during hard corners. Comes with all the connectors, the install instructions can be found on their website. The ASM technology works with the stock seatbelt system. And I think they are DOT legal.
it does not work with aftermarket seats. Below is the text lifted from the Schroth website. $200 shipped anywhere in the US.

The Quick Fit Harness from Schroth is an excellent harness solution for people who have dual duty cars (track and street). Not many of us have the means to support a full time track vehicle as well as a street car, and still more of us like driving track-prepared cars on the street. The ultimate harness solution includes fixed-back Kevlar seats, a gazillion point welded roll-cage and a HANS device. None of us are going to (nor do we recommend) do this in our street cars.

What's the next best thing? The Schroth Quick Fit Harness. It anchors to factory points, which means you don't have to worry about it ripping out of the sheet metal. The factory points are load tested to withstand a 3,000-pound force for 6 seconds. It has ASM technology that keeps the lap belt low and your rear end firmly planted in the seat. You don't want the lap belt riding up and squeezing your stomach and intestines. A Schroth belt is designed not to.

The Quick Fit Belts also take advantage of the factory seat belt pre-tensioner, a device that tightens the belt 1-2 inches in the event of an accident. The Airbags will deploy as the engineers of the system intended for someone buckled in. With any other harness, the car wouldn't detect that someone was buckled in, so the airbags would deploy faster and harder.

They also come completely out of the car, giving you full use of your street car for carrying people or cargo.