I want my computer to automatically disable polling of the CD drive when I 
switch to battery power; and re-enable it when I come back.

To achieve this, I think I have to use the "Configuration Notifications" 
feature of kpowersave (I use KDE) and add a shell script to be run when AC 
power comes and goes, which calls hal-disable-polling.

However, hal-disable-polling doesn't allow non-root users, whether logged in 
or not, to access it:

$ hal-disable-polling --device /dev/scd0
This program requires super user (root) privileges.

I think that that access check is too restrictive - the logged in user should 
be allowed to control polling of the drive.