This would be a bad idea, as then the applet would only be triggered if a GTK application sends a job to the printing system. The current applet listens to D-Bus messages from the CUPS daemon and from hal-cups-utils. So it gets triggered on jobs from any program, independent which toolkit it uses, if it sends a print job to CUPS all is OK.
"... or started in libgtk's print module."
This would be a bad idea, as then the applet would only be triggered if a GTK application sends a job to the printing system. The current applet listens to D-Bus messages from the CUPS daemon and from hal-cups-utils. So it gets triggered on jobs from any program, independent which toolkit it uses, if it sends a print job to CUPS all is OK.