More to the point, the current implementation of PP discourages people who are in Open from being in the same sector. People who play Open aren't usually the kind of people who enjoy Fortification, so basically everyone in Open is Undermining, like missiles passing each other in the air. And guess what, no is home to defend because there are no Merits supporting defensive combat. Apart from a few Combat CZ (Security Sweeps) this is fundamentally the exact same as playing Solo.
The only way I see this really working is if it somehow entices people to Fortify in Open, and if we also give some incentive for players who act as Military escorts for Cargo Fortification ships. This would allow for emergent defensive Wings in Open to be a viable strategy/tactic. Also as a back up mechanic, the NPC security in high Sec control systems needs to be johnny-on-the-spot with a rapid fire high power response to criminal undermining activity. Which means that PP NPC foreign agents that harass local traffic in core (high security) control systems should not be an issue. People shouldn't really be harassed by constant interdiction in High Sec systems by foreign agents anyway. That is as realistic as being chronically harassed by ISIS in downtown Houston.
I believe that is the point though that Sandro is wanting to deal with. Making people support in open by giving the bonus. As noted earlier though, he will need to find a way to make sure that bonus applies to only those who stay in Open from source to destination, otherwise its easy to get around with mode switching.
However, you raise a good point about those being in Open are doing undermining. We don't have stats, and Sandro says many thousands of people participate in PP. But how many of those are simply merit farming? How many of those actually care about the support side? How many people are actually taking part in powerplay and actually doing the (to my mind) boring grind of A to B deliveries and actually supporting their power?
The powerplay forums and subreddits have activity, but a lot of it is the same core players. Those who are really invested in the mini-game.
I'm not even convinced this proposed bonus will have the desired effect.
Hell, i'm beginning to think Sandro might as well go and do this (although i really dislike this idea of favouring any activities in Open just because of the potential for PvP and i'd really rather see development effort regarding powerplay to go into actually improving the mini-game itself to make it interesting enough to participate in). Because now i'm really curious to see if it will have the intended effect.