Some thought about the generated code that seem fixable. This would make people feel better about knowing that they can switch to the C code one day, and make it easier to debug problems in the generated code.

1.
It would be nice if functions were laid out like in glib/gtk+ code:
  static ReturnType
  function_name()

rather than
  static ReturnType function_name()

2.
There should be newlines between things such as functions, defines and enums. Right now it is all bunched up together like this:

void module_init (RygelPluginLoader* loader);
#define RYGEL_EXAMPLE_SERVER_PLUGIN_NAME "ExampleServerPluginC"
RygelExampleServerPlugin* rygel_example_server_plugin_new (void);
RygelExampleServerPlugin* rygel_example_server_plugin_construct (GType object_type);
GType rygel_example_server_plugin_get_type (void) G_GNUC_CONST;
enum  {
  RYGEL_EXAMPLE_SERVER_PLUGIN_DUMMY_PROPERTY
};
#define RYGEL_EXAMPLE_SERVER_PLUGIN_TITLE "Example Server Plugin C"
#define RYGEL_EXAMPLE_SERVER_PLUGIN_DESCRIPTION "An example Rygel server plugin implemented in vala."
RygelExampleRootContainer* rygel_example_root_container_new (const gchar* title);
RygelExampleRootContainer* rygel_example_root_container_construct (GType object_type, const gchar* title);
GType rygel_example_root_container_get_type (void) G_GNUC_CONST;

3.
Pointer type syntax could be like glib/GTK+. I don't like it (I like C++) but it's normal for C:

For instance, now I have:

  RygelExampleServerPlugin * self = NULL;
  RygelExampleRootContainer* _tmp0_;

(Notice the extra space on that first line).
though this would be more normal:

  RygelExampleServerPlugin *self = NULL;
  RygelExampleRootContainer *_tmp0_;

4.
There should be newlines between if() blocks and the following code.

5.
There should be newlines between the initial variable declarations and the g_return_if*() checks, and then between those checks and the actual code.

6.
This is harder to do right, but it would be nice if the lines were split, with an indent, at 80 (or so) columns.