A substantially different variant is to pay for every share at most once. If, when going backwards in the list of shares, we encounter some that were already paid, we skip them and move on to older shares. X needs to be less than 1 to make sure that eventually we will have enough unpaid shares to draw on. I'm not sure what's the correct way to handle difficulty changes with this variant.
Is this in effect what CPPSRB does? It pays all shares only once, starting with the most recent and going back as far as needed to pay out the full block (or until every share is paid). Your % of the block reward is the sum of all of your d/D since last block, where the total shares we're paying is = block difficulty of the block we just found.
Your comment on X being less than 1 to avoid an endless backlog is what caught my attention. Is the way I should think of that, if X is .99 then miners are making 99% of the reward and 1% is being kept back to pay shares older than the miners normally being paid. In effect, like a pool charging a 1% fee but paying that fee to past-due shares instead of it going to the operator?