Friend of mine that drives a Cobra Mustang had something similar to that at the last event. He said it works great but it's no replacement for a harness, as it only keeps your lap belt from adjusting on you. Also said it's a pain in the ass to adjust once it's on there, but that's kind of part of the design of the thing. He was only complaining about it because there were two different sized people driving that same mustang in one heat, so alot of adjustments were made.
You may want to have a look here... something I posted on another forum.
So on one of my many late night jaunts in Richmond when I had nothing better to do than attempt to satisfy my persistent need for some other maintenance item my car needed that I had handily forgotten on the LAST trip to Walmart, I came across a device called a "Mightytite Car Seat Belt Tightener." For a paltry sum of $10 (on sale from $16) I decided to try one of these out on an S2000 I was co-driving at an event. http://www.mightytite.com/Media/inMTlogohead1.jpg
This pretty piece of yellow and blue plastic combined with stylish polished steel slips over your seatbelt. It takes about 10 seconds to install/remove from the vehicle. Once you have your seat in the proper position, yank on the seatbelt to lock the inertia reel. Then use the ratcheting yellow handle to take the slack out of the seatbelt. You are then pinned against your seat, and as I found, you can crank the ratcheting mechanism so tight that breathing becomes difficult. This is useful in case you have a grudge against your co-driver and want to ensure he doesn't beat you. Release is easy since you can just use your standard seatbelt release button to get out of the vehicle. While this doesn't hold you in as well as a good 4/5/6-point harness, it offers most of the benefit for a much cheaper price. It is however MUCH less expensive, quicker to adjust for different sized drivers, and can be moved between vehicles in seconds. IMO, it works better than a CG-Lock since it locks the whole seatbelt against your body, not just the lap portion.
Mike, I think that the CG-Lock would be up your alley for locking the lap portion. The Mightytite has a ratcheting steel drum on it, so as you work the ratchet mechanism, it winds the belt around the drum. Since it slips onto the belt anywhere, it will tighten the WHOLE belt since it doesn't constrain the belt against the buckle. The CG-Lock in my recollection attaches to the buckle and locks the lap portion in place there.