For those out there who want to buy such equipement
look at page 48 of this link key-hole system
And Nacra has two sorts of harnas with a quick release system, but than you still have the problem of the metal rings flying in your face.
At our cat club there is someone sailing on an Nacra Inter 20 and he is very pleased about his quick release harnas,only one problem if you need to release it is difficult to put the plate with hook back on!