For gtkmenu based menus, there's gtk_im_multicontext_append_menuitems(). Since my app uses gtkuimanager menus, I need a way to create an action group containing actions for the input methods, and to create a menu merge ID containing the menu items at a specified UI path.

I propose:

GtkActionGroup *gtk_im_multicontext_create_action_group
  (GtkIMMultiContext *im,
   const char *unique_id);

(the unique_id would be used for the action names, e.g. <Unique>GtkIMAction<ID>)

guint gtk_im_multicontext_merge_ui 
  (GtkIMMultiContext *im,
   GtkUIManager *manager,
   const char *ui_path);

or maybe a combined method

void gtk_im_multicontext_create_ui
  (GtkIMMultiContext *im,
   GtkUIManager *manager,
   const char *unique_id,
   const char *ui_path,
   GtkActionGroup **group,
   guint *merge_id);

If that's agreeable, I can provide a patch.