Timeout for webkiosk

Hi all

I've started to use Matchbox in an application that it probably was not 
intented to be used. When trying to find a decent wm for my webkiosk 
project (http://www.edu.helsinki.fi/atk/ltsp_kiosk/) Matchbox caught my 
eye, since it is easy to customise. In this case, customisation means 
that you can compile it in a very stripped form and thus making the box 
a bit harder to hack. Also, the Matchbox binary is very compact and it 
requires no run-time configuration.

However, there is one feature that I miss. My users are logging in with 
their usernames and they are lazy to log out. Would it be hard to add a 
timeout feature that runs a magic command when no user activity has been 
observed for a while? Which parts of the code should I be looking?

t. Matti L