A three point harness should stop upward momentum if fast - the roller should lock and the lapstrap should hold you in. If not then I think this would be a fault or perhaps the lapstrap was too loose. It wont be as effective as a full harness but in my fast car days I actually practiced being turned upside down in a three point harness and not being a lightweight I was impressed by how easily it held me. (The point of the exercise was to release yourself without breaking your neck! - apparently very common in car accidents)
The key problem for me is that there is still a reasonable amount of play 2-3 inches so if your already near the roof you may be stuffed. Flying in the recent howling winds at one point during decent I lost the ability to acturately control since I was in being buffeted in mid air for some of it. Only lasted a short while at which point I pulled the belt tighter. (The cessna belt if essentially the same as a car - but with lumpy airbags)