I'm concerned that implementing this will only encourage more use of localStorage. The API is very poor because it requires synchronous IO and synchronization between browser contexts, which may live on different threads. (Note: Chrome does not implement the required synchronization.)
If we could fix localStorage to be asynchronous and transactional :-) then it'd be cool. Of course, one answer is that people should just use IndexedDB.
FWIW, Jorlow (when he was still working on chrome) expressed similar sentiments.