Piggyback on Bitcoin for independent data verification?
I think what you might be looking for is an append-only data structure for example "Balloon: A Forward-Secure Append-Only Persistent Authenticated Data Structure". A similar concept is used for certificate transparency as specified in RFC6962 

Edit: If you definitely want to use the bitcoin blockchain you should look at this or this.
Somebody actually has asked this question before.
A PoC implementation can be found here.