I think a slow ramp up in reward for those joining in the middle of a block would deter hoppers and reward the loyal, kind of reverse of what we have now.
Wait, do people not know how this pool scoring works?
Shares contributed at the very start of the window come easily, they have a low computational cost, and therefore, a lower score. Shares later cost more to calculate, and so have a higher score. Each hash is scored, the sum of all the hash scores added up, the block reward divided up (minus the pool fee) between all scores, and you get the proprtion of (your score / pool score) for a given block.
The only way someone jumping in at the end of the block gets the same reward as someone that started at the beginning is if that person has VASTLY more compute power available - and in that case, if they'd been in at the start of the block, they have gotten way more than the original full-time person anyway, because they calculate more hashes per second.