Currently Open/Solo solution makes pvp totally usless. Yes, you can kill someone, but why? For money, nah you can earn more by smuggling. Maybe in faction/territory controll/power play war? No, because it's hard to influence power play by pvp when players can play safely in solo. Also It will be impossible to create real system wars between players created faction because one faction can play in solo/group mode and grind grind and grind and it can't be stoped by another. It is plain and simple solo is carebear mode nothing more nothing less. Elite shouldn't have solos mode which can influance universe simulation.
There will always be some aspect of the issue you're highlighting here though surely?
ie: OPEN is not perfect... You could organise a group to "protect" a system for example (in OPEN), and other players turn up, and are simply in another instance! No different than had they been in SOLO?
I think the only way for thing like Powerplay to deal with this is to embrace the fact some people may be in OPEN and others not... and where there are competitive activities going on, they are accrued with an OPEN total and a non-OPEN total, and then combined. eg: With the OPEN results making up 60% of the final total, and non-OPEN making up the other 40%.
This would mean OPEN would always have a bigger say in the results, while not igoring those obtained in SOLO. In short, if you play in OPEN your actions will most likely have a bigger effect on any outcome...