What's the problem with knocking on the door? (have someone along for moral support, sat in the motor?) long time since I've done such a thing but met some lovely people by just knocking and asking about something in a field etc.
I'd agree unless it's someone you are aquainted with it wouldn't seem right taking a bottle, (just trying to think how I would prefer someone approaching me about land), I certainly wouldn't mind someone knocking on the door and asking, even if I would say no, but I'd be friendly about it.
saying is - 'if you don't ask you don't get'
another saying - 'a stranger is a friend you've yet to meet'
I'd go for it

, but talk about growing things before sounding them out about pigs
You'll be miffed if someone else just happens to notice the same patch while you're thinking about it.
Good Luck