With a D-Bus interface declared as follows:

[DBus (name = "org.freedesktop.Telepathy.Logger.DRAFT")]
private interface LoggerIface : DBus.Object
{
  public abstract async AccountFavourites[] get_favourite_contacts ()
      throws DBus.Error;
  public abstract async void add_favourite_contact (
      DBus.ObjectPath account_path, string id) throws DBus.Error;
  public abstract async void remove_favourite_contact (
      DBus.ObjectPath account_path, string id) throws DBus.Error;

  public abstract signal void favourite_contacts_changed (
      DBus.ObjectPath account_path, string[] added, string[] removed);
}

creating a proxy object for it works fine with the following code:

var dbus_conn = DBus.Bus.get (DBus.BusType.SESSION);
this.logger = dbus_conn.get_object (
    "org.freedesktop.Telepathy.Logger",
    "/org/freedesktop/Telepathy/Logger",
    "org.freedesktop.Telepathy.Logger.DRAFT") as LoggerIface;

but if the dbus_conn.get_object() call is replaced with dbus_conn.get_object_for_name_owner() (with the same parameters), valac tries to create the proxy object using dbus_g_proxy_new_for_name_owner() directly, rather than using the following proxy creation function:

LoggerIface* logger_iface_dbus_proxy_new (DBusGConnection* connection, const char* name, const char* path) {
	LoggerIface* self;
	self = g_object_new (logger_iface_dbus_proxy_get_type (), "connection", connection, "name", name, "path", path, "interface", "org.freedesktop.Telepathy.Logger.DRAFT", NULL);
	return self;
}

This function was generated and used with the dbus_conn.get_object() call.