Installing properties for arbitrary structure?

["Lawrence Nolin" <ln imap cc>]

To install a property for fundamental types such as integers, chars,
etc, you can use g_param_spec_*() and g_object_class_install_property().
My question is: how do I create a GParamSpec and install a property for
an arbitrary non-GObject C structure? I'm worried that values stored in
the properties won't be garbage collected. I could wrap the structures
in GObject subclasses and use g_param_spec_gobject, but that seems
overkill. Any help?