/usr/local/tce.icons gets rewritten each time you start the desktop and when loading extensions containing a *.desktop file.
You could f.e create a /usr/local/share/applications/<YOUR-WBAR-ICON>.desktop file and add it to your backup.
Or abuse the OnDemand feature by placing a script that executes your desired command, and a PNG icon with the same name, but with *.img file name extension, at /etc/sysconfig/tcedir/ondemand .
ICON: /etc/sysconfig/tcedir/ondemand/firefox.img
SCRIPT: /etc/sysconfig/tcedir/ondemand/firefox
Restart X to see changes.