Apart from the possible processing load, is there any technical reason why no pool uses a hybrid payment system?
e.g. 80% PPLNS + 20% PPS so that those who contributed during a round would get some value, especially for those who don't mine 24/7. However, this portion is not enough to be worth pool hopping due to the PPLNS portion which would encourage miners to stick out a long round.
Did you mean Proportional instead of PPS?
You're misunderstanding how hopping-proof methods work. They don't encourage anyone to stick out a long round. They just give the same average payout for future shares without regard to the past. So in a long round, the PPLNS part will give normal payment while the proportional part will give less than normal, so the total will be less than normal and a hopper will hop.
Hybrid schemes are possible but they tend to have the disadvantages of both components, plus the complexity disadvantage, without much of their respective advantages.
And, it is a myth that PPLNS has some horrible disadvantage for intermittent miners. They have the same expectation with only a slight increase in variance.
