I think is not possible, because the purpose of such legal measure is to avoid increasing ransom and kidnapping, and that would happen if the criminals knew that they would get the ransom from the coverage company. Hence any way to prove that the insurance exists goes against the legal reason to hide the existence of such coverage. To overcome this the following clause could be included: You are covered but if you communicate explicitely or implicitely to someone that you are covered then you are automatically excluded from this coverage.