I can totally relate to this, as my dad has an arsenal of heavy equipment. I never tied to the dump truck, but dammit now I want to try it!! I HATE when they learn they can break things whenever they feel like it. One of my trainer's horses is this way, and drives me insane. He could benefit from a dump truck patience session and a rope halter.
I can hear the conversation now...
*runNjump calls dump trunk rental company* "Hello, I'm interested in renting a dump truck for a day to tie a few horses to, what's the rate?"
Customer Service rep "...um....(silence)...aahhhh...(silence)...can you repeat that please ma'am?"
Anyway, I 110% agree that horses KNOW when they can get away with sh!t from certain people. All the horses on the farm know I'm not the Neverending Cookie Dispenser, and they also know when I say "Knock it off" in my mean voice, I actually *mean* Knock. It. Off. My gelding will paw for a few seconds, but he knows I won't glance his way. He spent hours tied up in his youth, and I can safely tie him anywhere. The only time he's actually gotten loose was last week...I put him on the patience tree to dry after a shower then come into the barn to clean tack. Lesson Girl walks in with Bailey in a halter that is not his and says "Uh, Bailey was over by so-and-so with no halter on..." I was loaded for bear...marched him back to the tree, thinking he had sat back and broken his halter, when I spot his halter still attached to the rope and his fly mask on the ground. The bugger was itching his face and somehow managed to get BOTH head gear off. Crisis averted, I re-attached him and thirty minutes later, he was still there.