It's coming! Existing SpiderOak accounts all contain a 3072 bit RSA keypair, which is reserved for this future purpose. But it's not as simple as it seems. Things like garbage collection, and space accounting, and various race conditions between parties become the hard parts.
In any case, it's on the way, and it's entirely open, built with Crypton.io, our new open source framework for building zero knowledge applications. (Which we'll eventually port the main SpiderOak desktop app to use.) For details https://crypton.io/ and https://www.youtube.com/watch?v=pn9DAwggza0 (...and to be clear, the threat model is intended for for HTML5 desktop and mobile apps, not browser Javascript.)
In any case, it's on the way, and it's entirely open, built with Crypton.io, our new open source framework for building zero knowledge applications. (Which we'll eventually port the main SpiderOak desktop app to use.) For details https://crypton.io/ and https://www.youtube.com/watch?v=pn9DAwggza0 (...and to be clear, the threat model is intended for for HTML5 desktop and mobile apps, not browser Javascript.)