To the OP I'm with you. I'd love to get a 4/5 point harness in my car (I lap almost every week) and slide around quite a bit. I don't want to change seats either as I like my OEM seats and also don't want to lose the heaters.
I've found nothing....
Certified High Performance Driving Instructor - BMW of North America, Porsche Club of America / Chief In-Vehicle Instructor - Fleet Safety International
Gone but not forgotten: 2008 E92 M3; Eisenmann Race Exhaust / Dinan 3.62 Limited Slip Differential / Dinan Software / Dinan High-Flow Intake / Dinan Camber Plates / Dinan Lowering Springs / Dinan Sway-Bars