Well as I'd imagine sending everything that has to be signed in a message to a private memory space you'd do the same for objects which need to modified. I'd believe you if you'd say that it wouldn't be very efficient and practical but from a security standpoint. If we'd be talking about private keys to big funds then I would let my private keys live in an encrypted vault like KeyChain on MacOS and just send messages to be signed through the sandbox back and forth. But in any case thanks for explaining, I should probably write up a post about it and research this topic. It's been years I've looked at these problems so...