Kut2-the-chase, that's the FASTEST way to get a "no".
I mean, think of it: How would you respond if a total stranger showed up at your door, holding out a contract for you to sign? Or you handing them a legal document, and talking about "liability"?
The only time anyone should introduce something like that, is if they themselves ask for it. Because otherwise, the only thing it does, is throw up red flags of things they should be worried about (and may not otherwise have cared less).
The best way is to 1) catch them "on their front porch" or other such non-confrontational situations. Because ........ we all know what we do when solicitors come to our door right? Say "no thank you" and close the door without hearing a word they say. 2) approach people as an amateur historian, doing research on their property (the "stage stop you heard was on their back 40", or the "swimming hole that was back in the canyon there", or whatever), and you were wondering if they knew anything about it. Have your pictures from the library book, or whatever you have. It helps if you are an actual docent or volunteer worker in a museum or something, so you can "name drop" that kind of stuff.
I'm a docent at 2 different museums (trust me, museums and historical societies are ALWAYS begging for help and community volunteers). And those credentials have gotten me into oodles of situations and places, even though it may not have ANYTHING to do with my metal detecting

The mere fact that you are this person of perceived authority, "doing research for an article you're writing", will open up doors. And then you slowly morph it into a ".... mind if I walk around and take some pictures?". And so on, and so forth.