I'm not an insurance guy, so I don't know for certain, but my understanding is there are companies who will write a policy for what ever you want, the difference being what you pay in premiums if you want something that isn't one of their standard packages.
Example, I've got my Grandfather's Ithaca shotgun. Been in the family since 1909. Say book value is $600, if I wanted it covered for that, its standard policy and standard price. IF, because of its sentimental value, I want it covered for $100,000 and I pay them enough money, somebody will cover it at that value, but I'll pay a LOT more than standard.
Might not be, and probably won't be worthwhile cost/benefit wise, but one could do it.
Best thing I can think of, is that if you have something with a unique value NOT listed in the "standard" books, you get that documented six ways from Sunday, if you can, THEN go insurance shopping with that proof in hand and see what each different one offers.
IF you go to one company's agent and they tell you they can't / won't write the policy you want (without even talking price), then you say, "thank you, have a nice day" and look elsewhwere. If its the same company you insure other things with, you might suggest taking that business elsewhere, as well, as leverage. Might not help, but if it doesn't, you're no worse off, right?