here is a different thread on this idea.
there is a diagram on post 25,page 3, and conclusions at last post.
Like I said before, I knew in the back of mind it must not work or someone would have already done it! Oh well. Thanks for the link to the thread.
You could probably tweak the design to work but it will be a bit fussy to use and as the others have said, the tension can be an issue when adjusting. The loop on the constrictor is not going really going any closer to the ridge line than the prusik loop is. But hey, mock one up and play with it.