Aye, Ian's right, you need to talk to your insurance broker.
My housemate's an insurance agent and I had the same conversation with him a year ago.
He recommended a particular company, name eludes my memory now, whose underwriters are more aggressive/willing. I'll ask him again later.
It's definitely custom and it won't be cheap.
So you need to figure out if it's worth it considering the total equipment cost.
I reckon if you're a professional, gear = work = income, so this isn't really something to skimp on I say.
Your home insurance should cover some, stuff from home that you may use outdoors, maybe possible to include some riders to that instead, but the best would still be something dedicated.
If you go this route, be sure to be meticulous about serial numbers and receipts; actually that's a good practice to have regardless.
The $500 limit you're referring to is what's offered by travel insurance packages.