Very interesting question. I've never boarded so I've got no clue. I'd definitely ask your BO first. You might also talk to an insurance company that does farm/livestock type policies if they could do some type of blanket type coverage for equipment.
I have an additional blanket for mine with our farm/home policy but it does cover off property when it's in the trailer that's also covered under the same policy. I will say that it is absolutely worth it to have that coverage. We had our barn cleaned out in '98. 13 saddles, dozens of headstalls, medical supplies, they even took halters off horses heads that were in stalls. The only good thing, all the show tack stays in the house. We recovered one saddle out of the whole lot. If it wasn't for insurance it would have been a huge struggle to replace it all. I still miss my old Cleburne that was taken then. Someone is enjoying that darn saddle, wonder if they enjoy that my name is tooled on it, jerks.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.