Why not buy it jointly? Otherwise you are telling people who bought the land what to do with it. If you want to have a say in it, pony up for it. Buy the land in the name of the township and turn it into a park.
"Externality" is one of the most tortured terms in widespread use and it would have been better if non-economists had never heard of it. Just because you don't like something, that doesn't mean anything has been "imposed" on you. You're dangerously close to describing the existence of other human beings as an "externality" that needs to be solved for, a view with a less-than-stellar track record.