Have there been attempts at a Bitcoin-powered general purpose database? https://en.bitcoin.it/wiki/Bitcoin-Powered_Database

As far I know, the Bitcoin blockchain is pretty much the only data structure that is both global and tamper-proof.

global - There is one global instance of the blockchain (up to forks ... which are then resolved).
tamper-proof - If a block enters the blockchain, after 6+ confirmations this block can't feasibly be faked or altered.

A Bitcoin-powered database would be an API over the blockchain, that would expose a subset of the usual CRUD database operations. In fact, it would be an append-only data structure, because nothing can ever be really deleted from the blockchain - so only CREATE and READ operations will be implemented. Object Versioning will be used to emulate updates and deletions.

Have there been previous attempts to create such a database? I have seen the MerkleWeb proposal, but I haven't really understood it, so I'm not sure if it answers my question.
This is it - the CryptoSphere.

It uses some Bitcoin concepts, and might support paying for storage using Bitcoins. I believe that "merge-mining it" using the Bitcoin blockchain would be possible (it's too soon to tell, the design is preliminary).