How about taking a jacket like this and changing the harness:
Kokatat Watersports Wear - ProFIT Tour
You would need to make sure that there are adequate provisions to keep the belt from riding up but at least on the one that I have(it is an astral but that wouldn't work as well for sailing), it is well sewn in in several places. You would probably also want to replace the quick release with something that you could clip your safety tether into but that would not be hard. I have rappelled off my jacket twice and it always did quite well. The only problem with this setup is that it won't keep your head up if you are unconscious. At that point though, most people would just be willing to accept it as an inherent risk to the sport.