Growing grapes downwards....

Dear all and particularly the grape growers amongst you 

I have been given two grape vines, Triomphe d'Alsace, which I am thrilled about. I was hoping to grow them up a huge south-facing retaining wall, which is about 12 foot high and 30 foot long and holds back next door's house and garden BUT closer inspection showed that this would a) ruin the patio and there is no decent ground underneath, having been filled in with compacted rubble. However at the top of the wall is a strip of land 2 -3 feet wide that runs the entire length of the wall, we call this the 'butterfly garden' because the buddleias have claimed it as their own and it is mecca for bees and butterflies. Would it be possible to plant the vines at the top of the wall and grow them downwards? Having spent hours on google, the closest I found is vineyard in Somerset that grow their vines along the top wire and let them flop down under their own weight before working them along the lower wires. Any info would be much appreciated, as always. We are on the south coast and the wall is in a sheltered spot that gets a lot of sun. Best wishes and thanks in advance!! x