I saw a system that Wouter had posted here a month ago or so, with both mid boom and regular sheeting. He said the F18 sailors are doing it by setting up the main sheet as normal but instead of tieing off the tail, you run it through a block at the end of the boom then forward to the mid boom block/cleat you have shown in the attachment. The system he talked about used a 2-1 set up which limits the amount of mid boom adjustments to 1/4 of your boom length, but if you don't use the 2-1 block, you could sheet from either end equally. That way, in light air you could use the mid, or use the regular for those "back of the boat" days. The "pull" of the boom would still be to the rear beam in either case.