PP doesn't need to be open only. For every PP player in solo/pg doing something PP related, the game can show some other player in any mode of play an NPC proxy that is doing the same thing.
If someone stops the NPC proxy from doing what it's trying to do, randomly cancel the contribution of one of the players who is doing that thing, but don't tell the player.
From the solo/pg players' points of view, they're still in their preferred play mode, and their personal narrative is unchanged. It's just that as a group, their contribution to PP is reduced by 1 action which is considered to be failed, regardless of the success or failure of the solo/pg players' actions.
There's still a reason for people to play in open, since in open they'll have a better chance of winning PvP than their AI proxy has of winning PvE.