I just submitted patches to the packagekit mailing list which implement a new
InstallPlasmaResources method in the
org.freedesktop.PackageKit.Modify D-Bus interface. There are patches for PackageKit itself, as well as for Apper (both master and KPackageKit 0.6.x) and gnome-packagekit. See my post to the packagekit mailing list for more details.
This completes the next step in my GSoC 2011 project. Now I can start working on using the new interface in Plasma.