Remember master password

It would be nice if `sdb` could remember your master password for a period of time, like `ssh-agent` or `gpg-agent`. Those agents won't work though, their protocols don't offer what we need.

ssh-agent protocol: http://api.libssh.org/rfc/PROTOCOL.agent
gpg-agent: http://www.gnupg.org/documentation/manuals/gnupg/Agent-Protocol.html

They will only decrypt with a private key, not store a symmetric key. 

Is there some way to securely stash away the password, or send a decryption request to some kind of agent?