I've finally managed to do the first release of ssh-agent-applet. I've actually been using it for some time on my own PC but it needed a bit of love and documentation to make it easier to install.
From the README:
ssh-agent-applet is a Gnome applet that allows you to conveniently keep your ssh key(s) on external media. This means that if your computer is cracked or stolen, the attacker will not have a copy of private ssh key(s).
Using ssh-agent-applet, your keys are loaded into ssh-agent as soon as you insert your "key drive" into a USB port. The drive is automatically unmounted once the key loaded so you can remove it from the USB port immediately.
For developers, ssh-agent-applet is a reasonable example of how to write Gnome applets in Python and how to interface with HAL from Python using DBUS. The HAL/DBUS interactions are non-trivial and go beyond what most documentation covers. It took a fair amount of experimentation to get some of this stuff working. Hopefully the applet or at least its' code is of use to other people.
ssh-agent-applet can be found on GitLab.