Limited - I'm just talking about people who forget to PM.
My only concern is the temptation based on the situation. I'm good with the honor system if everyone is okay with that, but consider the following. Suppose there is a face down card on an island and I would really like to gain control of that island. Based on other plays and where cards are, I'm guessing that the face-down card is a 2 or 3. Just to be safe, I decide I want to play a 5, but I forget to PM. The face-up card is revealed to be a 5.
At this point, I realize I forgot to PM and think to myself, "where's the harm in just playing a 6 instead"... In the end, I decide to be honorable and play the 5, but...
So far, it sounds like people want the intended play to happen, so I see three possible options.
1) Complete honor system.
2) Modified honor system. Basically, we give someone one "oops" and if they forget again, we figure something out that everyone feels is reasonable.
3) We let me flip. People can still keep playing if they aren't interested in the affected island, but I do the flips just to make sure decisions aren't tainted. I don't like this option as much myself just because I feel it might slow things down, but I can try to check in every couple of hours if it's the option that people would rather take. Personally, I think it's probably funner for everyone if they have the chance to flip their own cards.