I thought of it more as having the purchase attach in the centre of the bridle rather than slide along it. It effectively makes the sheeting point the windward attachment point of the bridle which lets you get the boom to the centreline.
Contact Kilwell Fibertube in Roturua, they are amazing to deal with. As for the sheet for the 'little back sail' (sorry I don't know where we are on the correct name for it) maybe consider either a bridle from each corner of the step, with the mainsheet purchase fixed in the centre or even...