I think the udev / dbus route would be the way to go i.e only having the keyboard appear when i.e a USB keyboard is disconnected or when the keyboard-dock is removed and visa-versa.
Running the above to probe a keyboard will not work 'on-the-fly':
'dmesg' (as well as 'hwinfo'?) keep track of disconnected devices and mentions them.
IMO the "best easy workaround" for now would be (at least for an E16 session ) to have 'onboard' in '.e16/startup-applications.list' to automatically appear and remain accessible through the systray icon.
Lightdm already has 'onboard' through the menu top right.
An issue does come up with an 'encrypted' (LUKS/LVM) boot where not having a keyboard will not allow you to log-in.
Where this project might be of interest: 
for udev, first try to run a command like "touch /tmp/touched", and see if the file appears when plugged, if that works, then you can continue trying to see if the "onboard" appears on the desktop, for that would be useful the new tool added in elive "elive-cron-run-gui" which basically makes something to have access to the graphical system (which of course you don't have from udev), also note that you need to run it from user so the thing to run should be something like this:
su eliveuser -c "elive-cron-run-gui onboard"
A last issue will be to know "which user" is the one that needs to run this (the active one, etc..) but this will be another topic
