Not sure how common, or binding, such an agreement would be, even if crafted by a lawyer. I'm not looking to get rich of this, just a fun hobby, and an old site would be one hell of an adventure into the past. I'd be thrill with the oporatunity, and pictures/video of the finds. The owners are generous enough to allow permission, it's likely they aren't going to want much of the spoils, specially if you don't feild clean, and dump everything in a paint bucket, for them to poke through at the end of the day. People are usually not attracted to dirty things... I'd just let them have what they want, the experience an memories are treasure enough for me.
You won't hunt it all in a day, if it's a good site, you will want to return. It's an old area, not just that one property, the neighbors, friends and relatives of the owners as well. Keep the greed in check, you might open up more ground to hunt nearby.
The historical issue, might cause some trouble. Some ares designated as significant, and the government has an interest, an agreement with the land owners, could be in the land deed, and not entirely up to the owners. The government was passing out grant money like candy, a few years back. Some people simply agreed to keep the exterior of a significant structure the same, in exchange for some cash...
