While I can't answer for after having scanned in and then leaving the area since I'm not nearby to do so, I can confirm as an AP with MP (didn't purchase it extra, I have a Sig+) that while I can see my MP benefit is there, I don't see a list of rides like others do once they've entered, I just see the "add to my party page" and then "not so fast!" page because no one in my party is scanned into the park when I click "continue".
So at least it looks like one has to be scanned in. I'll be curious myself to see reports on how far away one can be and still pull FPs through MP. Not sure I like that one doesn't have to actually be in the parks even after having scanned in. But I can see the appeal to being able to book during hotel breaks.
Or maybe there is a distance cut off somewhere since I am 400 miles away, haha! Someone will probably beat me to it since I'm not going again until possibly October, but on the day I drive home next trip I take, I'll see if I can still book further out (without actually booking so I don't take them away from anyone).