I don't think this would work on a lot of boats. The upper purchase block is often at the boom, and far above your head and normal range of motion. At least that is the way it is on my boat. If I had to reach up that high to cleat off the main sheet...it would never get cleated.
Yes... I suspect my boat's a bit of a special case. I am going to review the logic when I get the new Garhauer traveller in this winter (it replaces a stupid Harken "pin car" track that is not adjustable under load easily. If I can slide the bottom of the purchase to either side of the binnacle, then I can release from the bottom, but from the wheel.
My last boat had a cabin top traveller and a tiller and mid-boom sheeting. I could steer with my legs and adjust the main sheet and the traveller at the same time. The only issue was that as an old boat, it had a lid-type hatch rather than a "turtle": This made adjusting with the hatch closed a PITA, because the natural place for the spare line to go was "down the hole".
Neither of my boats has a boom vang, but I still know how to use a Cunningham.