To avoid kernel panick at resume from hibernate I have set
SUSPEND_MODULES="$SUSPEND_MODULES usb_storage"
in /etc/pm/config.d/00sleep_module .
I assume that reloading usb_storage will fail if I have mounted USB devices.
Is there a way to tell kde (e.g. by providing a script) to unmount them before going to hibernate. I could of course put some code in the corresponding /etc/pm script but then I would not get visual warnings about open files and hibernate would proceed in any case.

As far as I know KDE is not able to perform an unmount of attached media prior to initiating a suspend.

what about something with systemsettings -> application and system notifications -> manage notifications -> event source -> performing a suspension job -> run a command

question - does "a suspension job" include both hibernate and suspend?

At the moment the Power Management system allows to run a script only upon profile change, so I am afraid this is not directly possible. For answering another question, a suspension job includes suspension AND hibernation