What about Lock-On's and Climbing Sticks? Neither of these damage the tree.
Is it damage to their tree's they are worried about or is it a Liability issue? If Liability, type of a waiver such as

I Landowner name hereby grant Hunter name permission to access my land for the purpose of hunting at his/her own risk. I Hunter name agree to hold harmless Landowner name from any and all liability for any damages incurred to my person or property while accessing Landowner name 's property.


Landowner Name and Date Hunter Name and Date

Maybe this would give you another option.... Maybe you should do this anyway.I keep a couple of these in my truck and whenever I ask permission to hunt someone's land I take these along for the landowner's peace of mind and for my proof that I have permission to hunt that property.