I found out that
Action=org.freedesktop.udisks2.filesystem-mount;org.freedesktop.udisks2.eject-media;org.freedesktop.ud$
does the ejection as well.
Not sure about the last bit, though.
Source http://brahim.hamdouni.com/usb-mount-and-polykit-en/
Of course, it doesn't stop the disk from turning, but that's normal. I'd like
to check if the ejection really occurs also. I might ask the author of the
article directly.
So assuming this is ok, and understanding now that a netinstall demands a
manual configuration regarding policikit, what's left is to possibly
implement HDD poweroff with a command, and making it a shortcut in Thunar,
and voilà.
