If the "expected reward" per 1 diff share falls over time since the last block, then it is hoppable.
If you understand why Prop is hoppable then the above statement should be obvious.

PPLNS has a 'relatively' constant N diff shares, thus the "expected value" of any 1 diff share at any particular time, after the previous block, does not change.
(this of course isn't true over a diff change, but that's out of the context of the issue being brought up)

I can't make head nor tail of your description, but the above is what's relevant, and I imagine is what smooth is saying.
If your payout system does indeed pay a 'relatively' constant N diff shares per block, then it at least doesn't have the obvious Prop issue.