This proposal is very similar to Plan 9's "factotum" scheme (see http://qedragon.livejournal.com/99938.html for a nice explanation with reference to Heartbleed; factotum is similar to a generic ssh-agent or gss-proxy), except proposing that the daemon run as a separate user, which is a reasonable extra layer of security that deals with some remote-code exploits.
Yeah, I was aware of factotum when I wrote this post. GNOME uses p11-kit (which is a wrapper around PKCS#11) and gnome-keyring to kinda provide similar functionality.