I'm working on something fairly similar for Parsoid. We are mostly interested in revision storage, but can easily tack on a key/value store while we are at it. See User:GWicke/Notes/Storage#Key.2Fvalue_store_without_versioning for the key/value strawman.
The general idea is to spend some time on designing a good storage API, and expose that in a generic curl_multi based PHP class. A prototype implementation of of the revision storage part with Cassandra backend can be found at https://github.com/gwicke/storoid.