This feature was declined for F12, due to the need to port to GIO and none of that work has yet been done, therefore the feature is not in a testable state at this time.

for the records I have done the GIO implementation but it does not work due to a bug in GIO
as you can see in ML
https://www.redhat.com/archives/fedora-devel-list/2009-July/msg01960.html
I demonstrated that it's a bug in GIO

I have done a third implementation using system command mount
you can find it here
http://lists.freedesktop.org/archives/packagekit/2009-July/005191.html

by having three functional implementations I demonstrated how easy it takes to port it to any implementation

so how should I implement it

This looks like another instance of the "!PolicyKit refuses authorizations to root" problem. HAL will skip !PolicyKit authentication entirely (and always authorize the action) for root, it looks like !DeviceKit or whatever GIO is using there isn't doing that. As the !PackageKit backend runs as root, we have a problem there.