If I'm not mistaken, your API is designed to be used synchronously. Not sure how WebSQL or IndexedDB could work towards a synchronous API.
Overall localForage seems like a better cross-browser storage system, it also has a pluggable backend system (drivers) but defines an async interface.
I'm genuinely interested in supporting every needed use case - would you be up for creating an issue? Either way I'll have it in the back of my head and see what can be done.
Cheers, Marcus