lardman, Thanks!!
I wrote a script (seems wrong to call a two-liner a script) as follows:
#!/bin/bash
insmod serial_cs
I saved it at /etc/rc.d/rc5.d naming the file S49serial_cs
I believe the scripts in this directory are only executed if they begin with an "S" and some number. I presume that the number determines the order in which the scripts are executed, so I gave it a number higher than the rest. (After all it has been previously loaded after the zaurus boots completely.) Also don't forget to make the owner and group of the file root and make it executable, with these commands:
chown root /etc/rc.d/rc5.d/S49serial_cs
chgrp root /etc/rc.d/rc5.d/S49serial_cs
chmod 755 /etc/rc.d/rc5.d/S49serial_cs
Or easier yet, use TreeExplorer File>Properties>Security to change all that stuff.