Right, that is the fairest way.
I think for p2pool, it would have to be an accepted non-ideal thing. The total lost is 75 BTC (assuming 3 blocks) and only to those who are around if it stops.
The only way around it would be to set up some kind of trusted fund. For example, startup money could be paid into a 3 of 5 address where the 5 people are trusted. This would then be used to fund the wind down.
Having said that, I think if the shares count in terms of minting reward then the problem goes away for p2pool (effectively N never changes).
If you hit a block you get (mint reward) * (share difficulty) / (block difficulty) * (some constant)