So I've contacted the venue and here's the response about visiting the property.
We do charge $5 per person for groups of 6 or more using the picnic area. With that you get usage of our bathroom and tables reserved for your group. Groups are welcome to walk the trail without charge but we do need advance notice. You might also be interested in taking a tour of the house – the fee for that is $10 per person. We waive the picnic rental fee for groups taking the guided house tour.
Please update your RSVP with your intentions. I plan to walk the property, but I'd also like to cop a squat for lunch after. Please let me know what you're interested and I'll make the appropriate reservation with the property.