So you mean something similar to RSMPPS but also favor unpaid shares from miners who are still active. So if miner A worked on block 1 (a long block) and didn't get fully paid. And miner B worked on block 2 (another long block) and didn't get fully paid. Miner B stopped mining with the pool, but miner A continued mining. When the pool finally starts to get lucky and has enough BTC to pay the unpaid shares from block 2, we might favor paying miner A for his work on block 1 because he's still active instead of paying miner B for his work on block 2.
I think seems ok. It will give people incentive to keep mining and stick with the pool. But I can't think of a good formula that makes this simple to understand and implement. It might turn away miners due to it being too complicated even though the idea behind it is valid.
It seems similar to
MaxPPS, which pays you more than proportional on long unlucky rounds only if you have your own surplus credit from participating in short lucky rounds. This method was also proposed for Eligius, but it gained some negative publicity, because some people mistook its effects as withholding payments by operator.